We never claimed our MMU approach was faster, in fact it most likely is slower.
(Although the net-effect will not be very important.)
That's not the point however.
The idea behind the MMU approach is to leave the ABI/API unpolluted by the emulation technology.
Once the system is fully PPC native and all your apps and games are PPC native, you'd still be stuck with your legacy baggage in the ABI/API.
This is called thinking ahead. We're not going to go for short-term fixes.
Another reason is that it obviates the need to insert emulation traps into the source-code.