Often, looking closer to something while not actually being it can be worse than being right-out. Something that looks like a standard Mac app but isn't might annoy users who end up trying to do things in an entirely mac way, which will then fail. So one factor is that by looking like an X11 app, people will subconsciously assume it behaves like an X11 app, whereas with the "fake" aqua GTK+ theme people might subconsciously assume it behaves like a standard Mac App and get frustrated when it doesn't.
I only have the anecdotal evidence of Gimp.app for myself and my wife, so take this for what it's worth: The aqua GTK theme doesn't make it seem enough like a Mac App to fool people. It does make it look enough better that I think it would make users say "this X11 app isn't so bad after all". In my opinion the aqua look would bring only benefits.
If this is a concern, maybe there's someone on the list that could contact the Gimp.app folks and ask about the effects of the aqua theme. Maybe they could provide some before/after numbers or tell of any issues that arose.
Thanks, -- Michael Moore ------------------------------- www.stuporglue.com -- Articles, software and computer tutorials. www.stuporglue.org -- Donate your used computer to a student that needs it.