Harddisk speed has very little impact on your build time. It helps with the "setscene" parts, but doesn't affect actual compile time at all. I recall someone did a build from RAM disks only on a rig, and it was only about 1 minute faster on a one hour build compared to rotating disks.
My build machine has lots of RAM and I do builds in a 32GB tmpfs with rm_work (and no, I don't build webkit, which would make this impractical).
As you say, with sufficient RAM the build speed is practically the same as on disks due to the caching (especially if you tune the mount options), so I'd definitely spend money on more RAM instead of super-fast disks. I just prefer doing tmpfs builds because it saves my spinning rust. :)
