As sycamore has said above, there are a huge amount of variables that need to be taken into consideration.
It would probably take thousands of pages worth of posts to discuss "unlimited class" while a "spec. class" would be a lot easier.....but you still have lots of plane to plane variation.
Aseanaero has opened up a topic that cannot be answered easily.
There is a forum when many of the racers communicate.
Air Racin' - Aviation Airshow Air Race Photography Discussion