Grandiwell the power/weight -ratio is obviously the biggest factor in straight line speed. It's very track dependent whether a high power car is beneficial or not. Admittedly upgrades enhancing aerodynamical or mechanical traction can be unbelievably effective on twisty tracks.
power/weight has almost nothing to do with top speed, only accel. as someone said top speed is all about air resistence. the smallest area that the air hits and the drag that it causes.