I don't think so. I know the horsepower drops, but it doesn't take a big plunge to make that big a difference. I still think the cars get faster, because I recall going to the later races in a series without actually going through the championship and the cars would still be faster.
For proof, highlight the GT All Stars Championship in the Amateur League, but don't actually enter the championship. Instead, go to the first race (Laguna Seca). Now drive around at full throttle for a while then exit the race. When the replay comes up, go to the replay options and go to the "Item" section. Select the gas/brake meter icon (the last one in the second row, looks like two verticle rectangles). Notice how the opponents rev up the car slowly before the start. When the countdown ends and the cars roll off the grid, notice how the cars are not going full throttle. Switch between the other cars and your car to do a comparison.
Now exit out the replay and go to the last race (Tokyo R246 II). Do the same process as listed above, but this time notice how before the start that the opponent cars rev up alot quicker and more aggressive and once the race begins they are going full throttle.
