Well, what do you mean by "Aqua"? You mean the finder? Just get SnaX and replace the Finder with it. It's faster.
Or do you mean just ghe GUI elements and SFX? Well, there are tools to disable most of them, transparent menus, shadows, dock stuff etc. etc. but you will notice that most if this stuff is actually helpfull and not only eye candy once it is missing. Shadows for example. Try shadow killer and then again try to quickly indicate which window is over which....
It's the dilemma with EVERY app. I always would like to have more control over it, to deactivate what I don't like, but I guess Apple won't do that, it might confuse users. And even if you hide these options in some kind of "Root"-Panel, users might find it and might trash their system and then say OS X is a bad OS because they were able to do it. Such people exist...and they are many.
If only OS programmers could work a bit more like game programmers, where you can disable nearly every detail to get the game to a speed so you can run it. It would be great, still, I fear it won't happen...