If the carbs are stock then they are 26's. What is your elevation where you/your mate ride? When you said twin air filter, what do you mean?
If you don't ride in any mud or water, then I would ditch the airbox lid at least, or have him switch to two pod filters mounted directly to the carbs. If you do ride in some nasty conditions, cut 1.5 inches off the back of the airbox lid and you'll be able to gain more air, and protect the filter from "most" of the elements