Ahh, thats a tough one...
Thing is, that high compression pistons for the FS, raise the compression in the FP when used, but not as much. 10.7:1 pistons might give med the compression of 10.3:1 or something in that range in the 1.8 FP.
I've found pistons that give 11.4:1 on the FS, so I might...