I have no personal experience with rev kits. But back in the day when Ford thought it was a better idea, I read an article on their philosophy. IIRC, the purpose of a rev kit is to increase the rpm limit of the engine, hence the name "rev" kit. Accomplishing this higher rpm limit with stiffer valve springs increased the stress on the entire valve train whereas the rev kit only increased the stresses on the cam/lifter. If you wanted to maintain your current rpm limit and installed a rev kit, I don't see why decreasing the valve spring would be a bad idea. But why?