if its track days then its fine as there should be enough light. The F/2.8 is only for night usage or if you really want to create the DOF.
If i was you, work with the F4 IS and see what kind of shots you get and see if your happy with it. They say that the IS does bring an F-stop or more.
In terms of money, there is a big differance between the F/2.8 and F/4.