The change is actually to a smaller piston, not larger. That results in more pressure at the calipers. I drove EngineerMike's coach at Redlands after the change, and the brakes felt very good. But I didn't drive it before. Everyone who has made the change swears by it.
It is not a cheap modification, due to the labor involved in properly bleeding the brake system. The wheels must be removed to do it.
I don't know for a fact, but I would doubt any extended warranty would cover it.
'04 Alpine Coach 36' MDDS