Just to give another perspective:
I put a 240GB SSD (OWC Data doubler) in my iMac I7 Late 2009 on top of the 1TB spinning disk.
I have everything on the SSD except the iTunes media (~200GB) and the actual referenced aperture originals (pictures). Still >50GB free on the SSD.
The Aperture library resides on the SSD ~80GB.
When you work with Aperture it accesses the library and only if you want to edit a photo it will go to the spinning disk. (referenced originals)
This gives me a significant speed boost on my iMac! I would bet 99% of the speed of an all SSD setup. And the price of the whole operation was ~ $260.
I have 24GB of RAM to keep large files in memory for applying filters etc.
I think that is a really good point. It really is the thumbnails and the previews that benefit most from being on the SSD. This gives the opportunity to use "referenced masters" (or referenced originals). The masters (or originals) are not accessed frequently as you state. Hence... I agree with your conclusion that this is a cost effective way to speed up your Aperture library.
I do not use referenced originals. I have considered moving my originals off of my 768GB SSD and onto my Pegasus R4... and would clearly do that if I run low on SSD space. I like the "package" aspect of a self contained A3 library... because I periodically put a cloned copy onto my wife's iMac... and it is convenient. However... she just wants to "view and occasionally use" pictures... by copying them into non-Aperture projects that she is working on. This is just a convenience thing. I agree that I could get the majority of my current "high performance" by using referenced originals separate from my SSD.
Good point!
/Jim