Question about managing Aperture libraries
Relocate masters. First set up you new hierarchy on the external then relocate by project. I've never done it as I've always been referenced but that should do it.
Yes, you'll pick up a bit of speed culling on the internal. I don't think its worth it. You then will have to spend time relocating them again. My next laptop will have a 768, or larger ssd. My library will then be set up to keep "recent" files on the ssd and older files on the external. That way when I travel I have projects I'm likely to be working on with me. However, they will still be referenced on the laptop.
As a note, when moving images with the Finder, Aperture appears to keep track of this (to my surprise). Last weekend I completely changed the hierarchy for just about all of my image files. Changed hard drives, changed my mind in the midst of it and changed the hierarchy again. Out of 50,000+ images, Aperture upon opening had located all but 19 images. 3 clicks of the mouse and the 19 were reconnected.