My last sapph was stage 3 365hp, t34/48, on track at donnington I followed a white r34 round lap after lap, both cars seemed equal until we spoke in the pits, told me his was ment to be 600bhp and couldn't believe I'd stayed with him.
I know the track and car well but the best upgrade which separated us was my AP brakes, they were great until I boiled the fluid.
Not all about power as it's useless if you can't use it.