If they can take it, you'd need a DWBUA on a VAXBI bus (there is no Unibus adapter for XMI). ISTR when the 6000 was current that DEC imposed a limit on how fast a machine could take a DWBUA, not to screw the customer, but because some of the (then) newer machines had so many layers between the Unibus peripherals and memory that timings could not be maintained.