All of G5 I've seen, recognize automatically CPU-speed and configure themselves propper (Bus-Speed, etc.).
There may be a problem with older G5-mainbords, running only 1.6 or 1.8 GHz from stock, fitting with a 2.5 or 2.7GHz CPU-tower. But I never had done such a big leap, so I don't know, if they can do it or not.
Useing 2.3 GHz CPU on 1.8Ghz-motherbord works (Im writing at moment on such a G5). This also depends on installed Firmware. To know it for sure, means to test it......

If you install two new CPU's, both MUST be from same type and speed. You may try to do what I wrote above: Test it with only one CPU in place - you don't need more for MorphOS and OSX also runs with only one CPU.
(I also figured out that Northbridge becomes a lot cooler with only one CPU -> PowerMAC is running very, very silent!).