Mods not working?

I was running a McClaren in a GT career race last night around the Daytona Tri-oval. I had on the Super Rare mod that gave me +10% Power and +5% Grip along with two XP booster mods. About half-way through the race, in 4th place and not gaining on the Top 3, I paused and looked at the best lap times and discovered that the top 3’s best laps were about 2 seconds faster than mine. My top speed on the straights (and most of the curves) was 179 mph. Realizing I’d never catch up and successfully complete the race, I restarted.

I swapped out 1 of the 2 XP booster mods with a mod giving me +3% power and -1% weight and started the race again. With this mod equipped (along with the original +10% power and +5% grip), my new top speed on the straights was…179 mph.

Does power (and weight reduction) not affect my top speed? I ended up needing to crank down the drivatar difficulty one setting in order to successfully complete the race, as even running a perfect lap I was not fast enough to make the top 3 even with +13% power and -1% weight.

Weight reduction usually does not affect top speed.

There is a possiblility top speed may have been limited by aero or gearing but sounds a little suss.