GT3 lacks scalable AI competition. So it doesn't matter whether I enter a Daihatsu or an F1 in the Sunday Cup races, I'll be facing the same cars. That said, competition is, then, what you make it. You pick the car that you take in, nobody does it for you or forces you to make the choice you make. You pick the mods, you tune the settings, and you enter the race. If you want stiff competition, there is ample opportunity for you to get it. If you want to win by a mile, you can do that too. I, personally, prefer close races to blowouts. If there's no sense of competition, there's no fun, IMO. Make your choices and get the degree of competitiveness you want out of the game.
I do have to say that two of my pet peeves about GT3 are that you can get all licenses at once (should have to get one, finish all races with that license, then get the next, etc.) and that you win cars in events. Force ordered progression through the game and unlock cars instead of handing them out to make the game MUCH more entertaining and competitive.