I have changed the ring and pinion on an FX24D and a YM1720D for exactly the reason you stated.
The problem with the powershift is that you have to wait for the mower to stop spinning because when you go from neutral to drive it will catch and put a huge strain on your PTO
I would just buy a clutch and put it on, that way you will always be ready for another job, and its cheap insurance.
If you break the spider gears or ring & pinion you wont be happy.
Buy the clutch !!