the 400's are much faster. They are still, IMO, residential mowers. I know plenty of folks who use them commercially, and I also know plenty of companies who don't care of their equipment. THOSE--are the ones who always and consistently have to buy mowers every 3-4 years.
maintenance is key. If something is loose don't ignore it fix it. Valve adjustment is a requirement on the kawasaki and kohler as well. Keep the flywheel screens clean and make darn sure the air filters are serviced OFTEN. Leave the rockers run loose it'll beat up the top of the valves, the rocker arms, sometimes on the kawasakis the rocker arms will actually come off/loose from the head. Kohler is a little different design and that hasn't been seen at least not with my eyes.
I know plenty of guys who mow commercially, we speak often. The general consensus is that there are a few companies that dis Kubota for commercial use and there are a few that love them. Those that hate them? They have hired hands and their guys are minimum wage employees that couldn't care less about someone else's equipment. Ones that like them, actually take good care of them-including washing, blade sharpening, replacing worn deck wheels, etc-on a daily basis. Costs a little and takes time but it's almost guaranteed to prolong the replacement of the machine. One commercial customer I know of--has over 2700 hours on a Z421KWT, and it looks great. But he also make sure his stuff is maintained DAILY--meaning he don't run 500 hours on an oil change, and he don't let the flywheel screens get plugged solid with clippings.