If that was all, the Q switch upgrade alone would be a 3K+ upgrade - which it really isn't.
To start with, Series electronics have single-coil pickups with a 'dummy' humcanceller pickup. The preamps are powered by an external power supply, and operate at a higher voltage; I presume they are also built with to specifications. You can also have a stereo output (separate channel per pickup).
All other packages have dual-coil humcancelling pickups, battery-powered preamps, and mono output. The difference between Signature and Anniversary is that Anniversary has a mono/stereo option, and probably two volumes instead of a master volume with a pan control.
(Message edited by adriaan on June 02, 2008)