The appeal of the Amiga (to me) is reliving my youth a little bit. I never had anything faster than an '030 nor more than 16MB fast RAM. I've now got the same '030 but with 64MB (because it was cheap).
I could upgrade my Amiga to have a PowerPC CPU, masses of storage, a tower case, PCI board, Voodoo or Radeon etc. Utterly pointless though. I've got an enormously expensive and overpowered laptop if I want to do anything performance related (which, at the moment, means playing Minecraft). If I want a nice, retro experience, a 50MHz '030, 4GB CF and a 2GB CF in a PCMCIA adapter provide more than I really need.
A new, really fast CPU for an Amiga would be an expensive novelty.