for a lot of people used to playing with a controller playing with a wheel will be alot harder to get used to. but like anything practice makes perfect, and eventually you will be able to go quicker then you ever can with a controller.
it took me a good 3 months of solid use before i got used to it and started to see improved times.
when i built my cockpit and starting racing with reverse mounted pedals like in a real car i shaved another couple of 10ths off my times!
wheel is always better IMO, but sometimes not always practical. for a game like GT5 i dont think you have to have a wheel, the game was designed for controllers so the times can be beaten by a controller.