I have the same issue in a coupe but fortunately my car is a targa. There seems to be just enough space even with a helmet to be comfy. My car has factory manual sport seats.
My friend's 930 coupe with aftermarket sport seats and rollcage doesn't have enough head room with helmet. I'm 6'5" though...
I was also looking for some aftermarket seats for my car. Reason being the new PCA DE regs that don't allow the use of harnesses with stock seats. I think the Recaro Pole Position seats would be pretty good. The Porsche Gt3 seats would probably be ideal but they are pricey. Both seats are fixed though with limits access to the back seats.
I managed to cram 2 kid seats in the back for my twin 4-year-olds for short trips. Come to think of it, I really need a track car and a street car?! Anybody got a decent 951 for sale? Then upgrade the targa to a 993 or better.
jt
-84 targa