Upgrades come often, and unlike GT, you can go straight to higher upgrades. But with the big prize money and quick upgrades, the game tends to get easy. You have to be disciplined to avoid upgrading your car to the max if you don't want to simply blow away the competition. And that is the biggest problem with the import version. In Gran Turismo, racing had a pack mentality.
Races are always close and it's difficult to outrun the competition.
In Sega GT, racing in your class with a fully upgraded car is a snap assuming you can handle the car. Each car has its own performance limit, and as such the field thins out as you'd expect. If you put an underpowered car in the field, it will finish way off the pace. Fortunately, the more technical courses don't favor themselves to muscle cars. An 800 hp NSX is too loose to control on courses with many hairpins and S-curves.