I've done intake and exhaust work and tried a Dobeck tuner. It may have helped the low end slightly but in competitive racing my friends stock P700, we are still even.
There are some people that have done custom turbo jobs and make a big difference.
I have only heard of but never known someone to try a bore kit and high compression piston and cam. Some say that reliability declines when using aftermarket bore kits, cams, and Pistons.
Joe