I dont know about mulch kits. But you are right, I cannot fathom doing 300 1k sq ft lawns with "21's"... sorry I call them pushers too but lets not get into that...
Care to elaborate on that whole concept with the 21's? I would be interested to hear the reasoning behind that. Would your customers be resentful if you finished their lawns in 5 minutes? Or do they percieve larger mowers as being damaging to turf? Are operating costs less for your labor than for $6000-7000 mowers? Are you trying to keep employees busy so they are there and at your disposal for large landscape jobs?