So ... I ended up ordering a set of these as I like the way they look and was reassured on the fixing mechanism.
I decided not to fit the center three as I didn't like the idea of screwing self tapping screws into the body work of the car 'blind' for the center unit. Just seemed a bit agricultural to me and unnecessary since the center one is a blank anyway. Likewise the next two out as they aren't in direct airflow and provide useful access in behind the outer two.
The outer pair look very smart, but when you insert them as per the instructions they are under stress as they bend around the bar that goes horizontally across the aperture.
This pic shows them in place with no pressure applied. You can see them resting on the bar and also the gap getting wider higher up.
This one is pushing them into place, so bending them around the horizontal bar. You can improve it with pressure, but the gap is still there as the grill sits on the top of the vane.
The gapping around them is also 'variable', close in some places and quite wide in others. The gapping is better if you really force them, but then they are definitely putting pressure on existing parts.
They do locate in place firmly and wont damage the paint, but I am not convinced on how well they would stand up to repeated removal and refitting given you are bending metal tabs. I also think they will chafe where they are touching so are likely to mark the car. You definitely cant take the middle one off again if you've fitted the two self tappers without exposing the self tapped holes... and I really wanted something that wasn't going to mark the car at all, so if I chose to I could stop using them.
So whilst if I stood back I like the way they looked, I couldn't bring myself to fit them to my pride and joy as for me they fall short of the fit and alignment of the rest of the vehicle. My OPC had a 991.2 GT3 in when I was there a day or two ago and the grills fitted to that are night and day better.
It looks like the Turbo versions are fitted in smaller sections under the vanes and not over them and I think the GTS set would work better if it was the same. So a two part set instead of a single part that exerts pressure on the existing vane.
When I looked closer at other peoples pics of the GTS set I could see the same gapping and the same issue with them being under stress. I guess we all have different views on what is a good fit!
Anyway, Zundsport were excellent and refunded me my money with no quibbles. So a big thumbs up for their customer service. This specific grill set just isn't for me.