SkAiFeY,
I just realized that the reason the turns appear so fast to you is exactly the same reason the straights appear so slow. It's the absence of the unnatural wide-angle lens effect that I wrote about above. Because the field of vision is smaller than the wide angle that you are accustomed to, any lateral motion will appear faster.
And, on top of that, it's also because when you fix your eyes on an object moving across a monitor, it will not appear as clear as doing the same in real life. So maybe that's why you don't feel this adverse effect when karting. But the higher your monitor's refresh rate and the higher GP4's frame rate, the less of this adverse effect you will get. Are you playing at an adequate frame rate? Is your monitor's refresh rate 85Hz, at least? Also have you tried zooming out with the + - keys in cockpit view. That might give you closer to the effect you're looking for.
It will probably take extremely high frame rates to reduce these negative grahics side effects to negligible levels. Until then, we will have to live with these unavoidable compromises.