1) It depend on what you want out of it... if it's just faster gfx then probably not (although it's very fast compared to other Amiga gfx boards), if it's faster gfx and ethernet then probably yes and if it's faster gfx, ehternet and sound and more then yes.
2) Most likely, the BVision use CGFX drivers and Voodoo on GRex do the same and Voodoo on Mediator uses P96. There is software available that makes CGFX specific software run on P96 also so this shouldn't be a big factor.
3) That's 2 questions. First speed... I belive the GRex and Prometheus is a little bit faster than the Mediator. Though GRex only work with surtan unusual rev BlizzardPPC boards and it's known to fry itself and BPPC boards, the Prometheus is lacking drivers and requiers PCI extenders to fit normal cases, the Mediator has it's unsutanty around OS4 support... overall if OS4 isn't a must on your classic machine then Mediator is the best overall solution (don't know the status of GRex and OS4).
4) Mediator , no question about it.
5) Likely because the GRex has been discontinued and it's years old stock being sold out and no guarantees are left on the cards and they are known for their questionable quality... or could be because we're just willing to pay for quality hardware such as the Mediator?
I don't know where to get hold of GRex drivers but I presume you'll get that info if you buy such a card. Anyway, check these links for supported hardware on
Mediator and
GRex
