Well the D300 is Pitcairn-based (Pitcairn XT), so the equivalent card would probably be the AMD 7870 albeit with slightly slower CPU clocks. If you run it in Windows you can also use both of them in Crossfire X for faster gaming performance.
https://www.techpowerup.com/gpu-specs/amd-pitcairn.g420
The D500 is based on Tahiti (Tahiti LE), so the equivalent card would be the AMD 7870 XT although the GPU/Memory clock scores are slightly lower but it does have 3GB of RAM (instead of 2GB) and a wider memory bus. Windows can also use both in Crossfire X for better performance.
https://www.techpowerup.com/gpu-specs/amd-tahiti.g120
The highest end cards in the 6,1 is the D700, which uses the full Tahiti XT core (the same as the AMD 7970/R9 280X cards.) The D700 does have slightly slower GPU clocks, but the RAM is double at 6GB vs 3GB on the 7970. Again in Windows you can use Crossfire X to utilize both cards for gaming.
In their initial review, Anandtech tested the D700s out in Crossfire vs other cards including the equivalent 7970/R9 280X, where it was slightly slower (a 780ti is also there). I'm sure the D500 and D300 cards would also perform slightly slower than the equivalent cards in crossfire mode, if you search for some benchmarks of those cards.
https://www.anandtech.com/show/7603/mac-pro-review-late-2013/10