Somehow i find that hard to believe.
Making multiple logic boards for Power Mac's seems to be illogical from a cost standpoint.
That being said the G5 line from 2003 came in 1.6, 1.8 and dual 2.0 ghz
Referenced from apples own specification page at
http://support.apple.com/kb/SP96 created on june 23, 2003, four months before the creation of this G5..
Now compare that with the specifications from the June 9, 2004 spec sheet.
http://support.apple.com/kb/SP80
Still using the same FSB.. in the dual 2.0 ghz.. dropped the single 1.6 ghz in favor of the dual 1.8 ghz and added the dual 2.5 ghz. seems to me that the only thing that would possibly be a hold back would be the power supply, a connection on the logic board for the pump power/control and possibly a different style case to accommodate the larger footprint of the cooling structure of the 2.5 ghz. Also that spec list groups the 2004 G5’s memory capacity in the dual 2.0 ghz with that of the dual 2.5 ghz. There seem to be more than enough similarities to warrant me asking such a question.
Some hard facts behind your responses would lend credibility to you're postings.
Somehow I find my question not to be so far out in left field, I had hoped I would receive a response that was a little more involved than "no"..