replacing the "shock" forks for some cheap solid ones is the second best thing you can do to make your MTB road worthy. The first of course is thinner slick tires, and dont worry about the gearing. Chances are you won't be averaging 24mph right off the bat.
Actually almost no one uses the higher gearings on the largest chainrings when commuting. Even with MTB gearing with a fast cadence you can go pretty damn fast.
As a clyde who is getting heavier I will be switching to a MTB soon once I reach 235lbs. I am tired of breaking stuff on my road bikes.