Got my PRS-500 back a few days ago. Wiped clean. Lost my NIV Bible and City of Ember, but I'm not desperately bummed.
However, the PRS-500 is
definitely faster!! Not PRS-300 fast, but faster. This is subjective, since I can't side-by-side with my pre-update Reader.

However, I used the thing every day constantly, so I have a strong feel for the responsiveness, and that it has changed for the better.
Looks like the same screen to me, so I am betting there was a slight CPU boost. Which makes sense, given the age of the device... original CPU probably obsoleted, so they gave it a bump out of necessity (assuming they changed the CPU).
This might also explain the slight battery life loss. Faster CPU may be just slightly more power hungry.
-Pie