I have an old 48" Scag walk behind with side discharge that I use to mow my yard.  I put Gator mulching blades on it to help with leaves this time of year.  I don't have a mulching kit on it, just the blades.  I still put the bag on, and it gets full, but it is obviously chopping up stuff more and leaving it on the ground, which reduces the number of times I have to empty the bag compared to mowing up leaves with high lift blades on.  I'm happy with the arrangement for my own yard, but I've got a bunch of big leaf maples, so there's an obvious mess of mulched leaves left behind.  I don't know if many customers would be happy with the mulched stuff that's left on the ground if they were paying me to mow their yard.
I have noticed that the Gator's are much heavier, and when I engage the PTO there's a much more noticeable strain to get the blades spinning.  I don't know what issues that will give me down the road in terms of parts wearing out faster.  Hopefully it's just the belt.  I've also noticed a lot more fine particles of stuff getting packed up under my deck with mulching blades on, so I've got to spray underneath every time I mow.  I didn't have to do that with the high lift.
If you ever watch any Project Farm videos, he's done a comparison of mulching blades. You can find it 
HERE