Have a looke at my Revo;
I now have the RBTM523 in again.
I would not run 40 series tyres. They weigh so much more than narrower alternatives. I personally like the Crimefighter MTR's.
I have also removed the centre diff, they are not allowed in the MT class at my local club. I find the truck has so much more accelaration with just the FOC and now prefer it.
Have a read of the Steve Slayden Revo Performance guides on the Traxxax website, probably the best info available for a race Revo.