I think the logic of producing an updated Buster is that all our existing cards would benefit from a speed increase, as would 3000Ts and 4000Ts where there is no replaceable daughterboard.
Of course producing a completely new PCIe bus would provide better performance, but we're probably looking at a whole new machine at that point - and several efforts (NatAmi, other FPGA Amigas, etc.) on that front are already underway.