When i've played GTAT it wasnt bad, except for the cars floating in air or invisible cars(which has been fixed). If he just got the mouse wheel working it would be a lot better. But the chat box is a little weird, unlike MTA when you go to type all the buttons still do what they would normally do, like change radio, get in/out of a car.....blah blah blah. He could really make this a great mod if those little things were fixed.
EDIT: Also when i played it Zircon was playing too (56k'er) and the lag wasnt bad. It looked like Zircon had about a 10-20 ping on a MTA server.