You can get some excellent roll out on the right 26 inch tyres... it is the improved aero position that makes a world of difference and in my case my MX is nearly as light as my steel framed road bikes as it too has a very lightweight racing frame (lighter than many road frames) and all the weight difference is in the wheels.

If I ran narrower or higher psi tyres this bike would beat me to death as it is stiff with a capital S.