I am too lazy to dig up my links/writeups on this.... Yes, you can use the 2.3ghz CPU set. Manage to bring a 2.5ghz cpu set from the dead by scrubbing them clean, but no guarantee on this either. Some G5 2.0GHz setups can clock up upwards to 2.5GHz or even 2.8ghz, just depending on the board's firmware version. Used both heatsinks from the 2GHz onto my 2.5ghz and 2.8ghz set, but also swapped out all the fans so they run the whole time. So this way I don't have to worry about burning up the CPUs.

Beware, the fans will run at full bore once you swap them out, but you can try your luck at the Apple Service Diagnostics CD. I did manage to get the 2.5ghz set to calibrate to the mainboard. The 2.8ghz set I couldnt get to calibrate and would error out. This was when I decided to replace all 4 fans with my own straight into the power supply.

[ Edited by Doffo 19.01.2014 - 00:49 ]

I have been reading some of your post. Very interesting.
If I understand correctly, if I plug a 2.0 GHz cpu to my 2.5GHz system, it will clock to 2.5?
Will have heat problems without the liquid cooling system?
Will I need more fans?
Now I have an unusable computer so I dont know what firmware version I have.

Still reading your posts....thanks!

I think it'll run at 2.0GHz.

Ok...I dont really care. Only want a working machine.

Correct, the CPU sets themselves set the clocking. So a 2GHz set does work on a 2.5GHz board. Just pull the whole LCS out and use your stock 2.0GHz CPU sets.