Or someone else improves the other 68k softcores. It's not like vampire is magically the only fast FPGA 68k emulator.
Apollo likely has a headstart, but you don't know what people are doing in private.
E.g. the MiST is about the level of a 30 - 35 MHz 68030 and it's running on a Cyclone III, same FPGA chip as e.g. the Vampire V2 A600. Of course the MiST recreates more than the 68k core itself, but so does the Vampire V2.
The Minimig-AGA core has been developed since 2014 at least, so it's strange that performance hasn't improved. Maybe all the focus has been on improved compatibility.
If you're not using Vampire AGA or Vampire PAULA, the difference between a V1200 and a BPPC + BVision is none regarding this (except for the PPC part). 
It's still not a "real" 68k processor though. I get your point, but for me personally, an FPGA emulator strapped to an Amiga does defeat the purpose of using legacy hardware in the first place. If there's a compatibility issue in the 68k softcore, AGA or Paula, it would affect both the standalone system and the "accelerator" equally so I see no real advantage to not going with the standalone board. I guess you could mount it in your A1200 case if you wanted...