"Steve, are you saying hold the nut still with a box wrench (from underneath is the only sideways exposure), and then use the impact gun or driver (from above) to drive the shock top clockwise (therefore effectively loosening the nut counter-clockwise) ? I'm doing all this via the top mounting hole in the fender because I don't seem to be able to fold the whole shock tower out of the wheel well with the control arm and sway bar etc. still in place."
When I did my H&R/Bilstien conversion, I unbolted the sway bar from the wheel carrier, used spring compressors to compress the spring. I then reached up into the wheel well and pulled down on the camber plate, compressing the shock. You may need a helper for this. I then had my helper push down on the brake disk to drop the wheel carrier as far as it would go, and swung the shock out of the wheel well. You just have to be real careful about the shock slipping and extending as you swing it out because It can do some scratching to the fender if you're not careful. Once the shock swings out, you can relax everything and you should be able to easily pull the shock from the wheel carrier. The only extra step for the rear is disconnecting and dropping the inner drive shaft CV joint(this will allow you to push the wheel carrier down in the rear.
You can try it your way, but I'm having a bit of trouble visualising it. I say give it a shot, if it doesn't work, I know it can be done off the car as I suggested. Quick question, how are you going to compress the shock to get the camber plate out once you have everything loose? You may find it kinda difficult to keep the shock compressed to slip the plate off/on.
If you go to the Boxster wikia, you can see the instructions for doing this in my wheel bearing R&R section. Step 4:
Boxster WikiaHope this helps you out...
Steve
Steve
Guards Red 1999
I'm not a race car driver, but I play one in 2nd and 3rd gear