I've hit 336.2 mph without using the wheelie or any type of cheats. I already read that someone got 340. Congrats!
My cousin showed me how to do it. The best I could get was 278 mph in a fully tuned Lotus Esprit. He was getting 301, 307 & finally 317 mph in an Escudo. He told me that you have to roast the tires.
It went something like this:
My cousin: "Roast em."
Me: "Okay." (Half hearted attempt at burning the tires through 2nd gear follows.)
My cousin: "No. ROAST EM!"
I then did the mother of all burnouts all the way to 299.9 mph! The tires never stopped spinning the whole time. There was a long trail of white smoke following the car all the way down the track. It looked like Wile E. Coyote riding a freakin' ACME Rocket!
Eventually I beat all his best runs and ended up with 336.2 twice. I've also got 301.1 mph with a Stage 3 turbo. That's about 400 less hp. I didn't do much testing with this though.
Oh, and by the way, my cousin ended up with 335.7 mph for his best.