I've asked the same question elsewhere. I have a PIV/Z4 bus system, the PIV running as Zorro2. I'm about to fit a Mediator/VooDoo5 and wondered if the graphics improvement would be worthwhile.
The answer was yes. It's a dream on wheels compared with Z2. However, it's not likely to be faster than a PIV/Zorro3 setup. But then 64Mb will always beat 4Mb, eh?
JaX