I'm not sure what I can add but I'm in a similar boat but my driving force is better control and a more planted feel rather than lowering. I'll offer my thoughts so far on what I've found in the hope it assist in some way. Suspension is very subjective but to give an idea on on my scale of acceptability for road use: -
1) My E46 M3 runs Bilstein B12 (B8 inverted mono-tube dampers matched to eibach lowering springs). This is quiet jiggly below 30 mph and not great over pot-holes/speed bumps/roadwork patches etc, too harsh, above 30mph it really comes alive and flows nicely with the road and controls the body well, turn in is really quite nice. Eibach springs alone were far too soft, OEM was far too soft and too much roll/slow turn-in and the rear end wobbled about a bit on power corner exit.
2) Celica GT4 is running a very firm coil-over setup (100nm front /60nm rear), it's not ideal. When you back off damping for road use it struggles to control the car over road undulations properly. With higher damping rates it controls the body but is too jiggly for most roads unless you are 40mph +. Definitely a high-days and holidays car. It can activate the ABS over road ruts. Really only suited to smooth tarmac.
I assume you want lowering for a visual improvement? Generally lowering will make standard suspension worse at handling bumps and maintaining alignment as you have less suspension travel to soak up bumps and unless you spend a chunk of money on additional alignment products; you move the roll-centre of the car, suffer more bump-steer and may struggle to get good alignment over the whole travel of the shock. When I spoke to Centre of Gravity they suggested uprated ARB's over lowering springs unless I wanted to lower for looks, in which case they suggested Eibachs. On looking up spring rates, both H&R and Eibach are similar rates and only around 10-15% stiffer than OEM but the Eibach are progressive so start softer (than OEM) and then get firmer the more they are compressed. Teh general suggestion seems to be only fit lowering springs for a better look/worse ride/handling and if you're on a budget.
Some of my observations have come from a skewed starting point I've recently found. I'm in the middle of replacing a front snapped spring and have found play in arms and I suspect a big alignment issue where one strut top is full inward and the other full out (A curb bump prior to my ownership?) I've tried to get the car aligned twice during my ownership at local places but both times been let down.
On purchase of the car I found normal suspension mode too soft and Sport mode far too firm. Reading the internet suggests that standard is 20% damping and Sport is 80% damping so hardly surprising it's too firm. What I did notice is that in sport mode the rear end wallow was generally removed and the car felt more planted. As such my first step was the DSC PASM controller upgrade. This effectively sets the normal mode to 10% damping and it increases damping % upto 100% as g-force increases (due to cornering or acceleration/braking.) the Sport mode has a range of 20% to 100%. You can tweak these settings though on the map. This effectively allows the car to absorb most bumps and ruts when you are cruising or pottering about your village but it firms up as you push harder.
My thoughts on DSC were initially very positive, sport mode is now as comfy as the original normal mode, IMHO if anything the normal mode is too soft. Sport mode generally corners flatter than the original normal mode and inspires more confidence. Occasionally the system does get caught out IMHO and in part I suspect due to the slow reaction time to change damping rate of the OEM dampers and there being too much of range of damping forces used in 1 specific map (e.g. 20-100%). A combination of a low but large undulation in a road followed by a sharp corner (with or without more undulations) really unsettles the car IMHO as the damper is still set to the "soft damping %" of the straight ahead when you enter the corner and it can't firm up mid-stroke with OEM dampers. I suspect I could dial out most of this issue by increasing the minimum damping %. We also have to consider that the 997 has 1st gen PASM and later generations have position sensors for even more control by the DSC which isn't available for the 997 so maybe the DSC isn't as intelligent/effective on 997 generation cars?
As I'm replacing front shock absorbers (which I suspect yours will be corroded as well and require replacing) I'm looking at options (major mission creep).
I've now been out in a couple of passive setups: -
KW V3: - this was set to track damping and was evidently very firm for the road (too firm in the owners words and he usually reduces damping a lot for road use). It did however totally transform how the car felt, it was incredibly communicative, far faster to respond to inputs and felt like a sports car rather than a GT car like a standard 997 does. I'd like to experience it on softer settings for the road, I couldn't live with it on the settings I experienced.
Ohlins R&T: - This was running a damping setup that I believe was for a mixed fast road and occasional track. It was far more compliant than the KW but still quite jiggly at low speeds. OK for a weekend car but too firm for a daily IMHO. Certainly a fair bit former than my M3/B12 setup. I didn't crash over bumps though. I have read that the Ohlins DFV valve effectively blows off pressure during big-hits to smooth the ride, this has been suggested as being a good option for road use but upsetting the car when on track hitting curbs.
Pretty much all coil-over setups run around 40nm front springs and 100 nm rear which is similar to 997 GT3 OEM spring rates. (whilst the difference appears massive, the "wheel ratio" of the rear damper is different as it is more in-board on the suspension arms so the resultant difference at the wheels isn't so great).
As such I think I need to retain active damping control as I want mine to be able to daily and for weekend blasts and maybe even an odd track day. My choice therefore seems to be Bilstein B16PASM or Tractive Touring line, neither of which I've experienced. There is quite a big difference between them on cost and technology.
Bilstein B16 PASM is effectively what the GT3 comes with, you can retain PASM and add the DSC controller but it is still slow to react and can't adjust damping mid stroke. Reportedly much better body control and still can be compliant.
Tractive is around 20 times faster to react and can change damping mid stroke. Less likely to be caught out by complex road condition I suspect. I've not read anything bad about this setup.
The final 2 "new to the party" options are KW now does a setup with in-car adjustment of damping rates (not active) for the 997 and Ohlins has a range similar but yet to be available for the 997.
How much mission creep can your wallet cope with?
