Once again people are making the mistake of thinking that any good looking game on XBox uses every ounce of its power. The argument from some people often seems to be:
'This game is on XBox, XBox is a bit more powerful then GameCube, so a GameCube version of this game would have to be scaled down."
Its a majorly flawed argument.
From what I've seen Panzer Dragoon Orta isn't that fantastic looking. Its not a step above something like Rogue Leader, maybe not even as good, (I don't know for sure as I haven't played it yet). So I see no reason why the graphics would need to be scaled down.
I'm quite confident that XBox and GameCube have allot more in there hardware then Orta is doing.
Ozy
You mention that Rogue Leader was a launch game and is still technically the best looking game, not just on GameCube but still one of the best on any system IMO. Well that shows what can be done with the GC hardware when a dev really knows what they're doing like Factor 5. Rogue Leader, put together in 9 months, is right up there with games like Orta, so you really have no reason to believe that anything would have to be scaled back for Orta.
The only way I can see what you mentioned happening, dropping texture detail/polycounts, is if the game was just a direct port from XBox to GameCube. But then I can also see Rogue Leader being worse on XBox then it is on GameCube with a simple direct port.