Is that true, though? I've heard that if you combine foods you can get a different GI than if you eat them separately. If that's true, then it seems like you should be able to pull off the faster-absorbing amino acids and supplement them on their own in order to maximize their absorption rate, without being laden down with the slower-loading amino acids. Essentially, cut it down to what your body can use ASAP and don't divert any resources to the slower stuff. After your workout or during normal meals it wouldn't matter, but in a workout situation I can see time sensitivity being an important matter.
But I could just be totally misunderstanding, or full of bad information. It's happened before. I started this thread exactly because I don't really know!
I think you are mixing two concepts. Different macronutrients affect gastric motility in different ways. The sum of the influences determines motility at any point in time. If there is more fat motility slows down, if there is more carb, it speeds up. But that affects everything that is in the stomach at a particular time in the same way.
Absorption is a different matter.
But basically, if some AAs get into the blood sooner, they just have to wait around for the others anyway. I think Chris has it pretty straight.