I want to slap all of you upside the head.
While it may not be a good idea to allow mods right now, I can't see why not allow them in a final release. Look what they did to NASCAR Racing 2003 Season or Grand Prix Legends, for instance.
In NR2003, you got leagues running all kinds of mods, from Aerowar 88 to Grand National 70 and the current COT cars, on all kinds of modded tracks. Some car mods even have their own physics, which are, for most of them, pretty good. I can't even bear to play the vanilla game again.
In GPL, you just need to download the 2004 demo on BHMotorsports and compare it to the original game. The 2004 demo is a compilation of all the greatest mods released for GPL, and it looks and feels like a completely different, much improved game.