Question: Do you even wind up racing against that infamous blue 240z at the end of Street Supremacy? I don't think so, not unless (like the latest trend of TXR titles) you wind up beating every single rival and wanderer in the game... but that's just a thought in my head.
I don't think you should worry too much about wheelspin, since you're running for top speed, not times (although it IS probable that wheelspin may hamper your capacity to get up to speed in certain time attack sections of track, most notably Route B (Wangan) past the Yokohane exit: that loooong loop around back to K1 and Yokohane northbound).
I'm assuming you're doing top speed runs using Time Attack just past the Haneda and 13-Gochi interchanges (i.e. the part of the Wangan that is the absolutely FLATTEST and STRAIGHTEST section in the Tokyo Bay). How tall (or "top-speed-friendly") is your first gear? I'm just guessing here, but the key to unlocking your potential could be resting on that first gear.
If we're strictly talking top speed and power here, I would imagine first gear would work out for your experiment. Try to see what happens if you make first gear as tall as possible, and come down to your second and third gears (i.e. second gear TALLLLL, third gear TALL, fourth gear not so tall).
The wheelspin may come in handy: if your first gear is ridiculously tall, then spinning wheels under power (especially when tires wearing out is not a concern
) would help you build up speed and carry you into the powerband.(Again, talking about making your gears taller is an assumption that it makes your car go faster.)