Adding a late candid question but; in fact what GPU specs really matter for MAME in general, and GroovyMAME specifically ?
For instance here are the three discrete GPUs I own at the moment (all very similar except my particular 360 which is factory boosted so this doesn't reflect its performance);
EDIT: to expand, is it only core clock, or does something like 2D performance also count? because most modern GPUs have outstanding 3D but suck at 2D on Windwos OSes, and especially modern AMD cards it seems, some old models like R5 430 were much better for that. Even some Intel HD iGPUs outclass expensive AMD and nVidia in that area.
I don't know much about GPUs but looking at comparisons, if I was looking for a clear upgrade in every field and something more or less available on the market, I'd have to look at a R9 390/X, RX 480 or RX 580, everything below would have either lower core clock or lower 2D performance, or just too little performance increase scores (save for 3D of course) to warrant the price and and higher power requirement.