You guys need to give the game a break. When you fully mod it try changing the gear ratios and you'll be running fast. Also testing on Arcade mode is not the same.

In GT3 i had the Quattro TT running 12's. The game is not only modding and racing. Tune your car. Tune it to accelerate fast (sacrafice top end).

Also I player Forza(Demo) last night while waiting for GT4 at EBGames and it sucked.