After some careful work with the dremel, I've finally managed to fit my new Radeon Pro Duo in to my MDD. It takes up a fair bit of room in there, and I've had to run it off a second PSU (currently sat on the floor behind the mac, with the wires going in where I've left out a PCI blank, I'll have to improve that at some point).
Runs my 2 new Samsung 8K monitors really nicely, handles both at full resolution at 60mhz easily. Going to need a bigger desk next, they're wall mounted ATM, which works I guess.
Might try overclocking it at some point, to increase the frame rate.