Right, I thought I'd reply to this as used this thread to try and figure out what Group 1 seat (the one after rear facing 0-9kg) to use in our 996 Cabrio. And I had a right struggle!
First point to re-iterate, the rear bulkhead in a 996 Cabriolet is different to the coupe. It is further in due to roof mechanism and is also steeper. This precludes a lot of seats and also precludes removing the seat backs a lot of them time as this then leaves the ledge jutting out and a huge gap between seat back and bulkhead.
I pre-ordered the kiddy energy pro as these apparently fit. I could not get a good fit with these or the Guardian model. The staff at a specialist tried again today and also failed. I couldn't get it to seat decently in the base so it never actually felt secure (with/without the seat bases/back removed)
Yesterday evening at Mothercare we tried the Britax Duo plus and the Britax Prince. Neither of these fitted properly and we couldn't safely secure them (sorry, should have taken photos of all these attempts!). The lady there also reckoned she had no seat in stock that would work with the cabrio.
Further reading then suggested that the only guaranteed fit (that doesn't compromise front seat position) in the 996 Cabrio is the Nanio Trio Plus (now discontinued and not exactly glowing safety report from Which), the discontinued Porsche Prince (only available on eBay) and then the 'Britax two way elite'. This is not sold in the uk by Britax so it's re-imported from Sweden by one company in England,
http://incarsafetycentre.co.uk/product/2-way-elite-black-thunder/ .
By sheer coincedence, one of their three branches is 5 minutes from us and also, 1 of the only 2 Porsche Prince seats on eBay was also 5 minutes away! In my opinion the Porsche Prince isn't a good fit, it's designed to fit perfectly in the coupe so, again, doesn't match the steeper bulk head on the cabrio. Also, it's basically just a covered piece of polystyrene with an abdomen bar. With some hassle it could be wedged in but you would have to do this every time you put the child in the car.
So, I've come to the conclusion that the ONLY seat the fits the 996 cabrio back and doesn't impinge on front leg room (like the maxi-cosi tobi does) is the Britax two way elite. The only downside which I don't think you can get round is that they are very upright so if nipper falls asleep the heads slump forward.
Sorry, need to add...the discontinued mamas&papas protec is mentioned lots but I believe doesn't suit the cabrio because of the steepness of rear bulk head..