Tire type is extremely important. In fact, I'd say tires have a bigger effect on performance than any other part in the game.
In practice mode the tires don't wear out - it would be frustrating to be trying to do a bunch of laps and to have to pit every few laps. Also it's no fun when your grip levels change so much that you only really have an opportunity to set a record maybe four laps out of ten.
I dunno, I think Arcade mode is better for these tests. That way you know everyone is driving exactly the same thing. But if you want to continue with GT mode, feel free. You should just specify every detail, like which mods (you said racing brakes), which tires, whether you did an oil change or not, and so on.