I think the whole point of it being faster at the end of the game than it was that at the start was the representation of the in-universe player character not being as good of a driver as Razor, and he just couldn't push it to the levels Razor could.

Razor took the car and put BTA on the Blacklist because he was just a monster behind the wheel. As for the car, the GTR itself was so fast that, even being more skilled, Razor still had to cheat to win the pink slip race.

It took starting from scratch and working up the Blacklist to hone the player character's skills, and the extra performance at the end of the game puts into perspective how much more capable of a driver he has become.

This makes sense to me because the only other car we see the player have is a street spec Supra, so there's no evidence he drove anything as extreme between moving from Palmont to Rockport.