I've had one for about a year. Really like it, but it's no target rifle. Mine will shoot a wide range of bullets and powder charges with decent hunting accuracy (3" five-shot 100 yard groups).
I've shot 385 and 410 grain Great Plains conicals, 250 grain Lee R.E.A.L.s, and 250 & 300 grain Speer Gold Dots, 300 grain Hornady XTPs, and 300 grain .458 Remington SJHP's in about five different brands of sabots, with every powder charge from 80 grains to 120 grains of Pyrodex and Triple Seven. I can find at least one load with each of those bullets that will shoot 3" or 4" 100 yard 5-shot strings.
My absolute best load so far is the 300 grain Speer Gold Dot in a Harvester short black sabot over 100 grains T-7 FFG. This load will give me consistent 1 & 1/2" 5-shot groups at 100 yards. Average velocity is 1755 fps. I swab between shots with both sides of one damp patch and both sides of one dry patch.
Good luck with your Mustang. Give us a range report after you get some shooting under your belt.
(Add On) I see no reason you shouldn't be able to find a good load with your GOEX. I checked my records and have only fired one group from mine with GOEX (It's hard to get in my area and I'm saving what I have for my flintlock.) I shot three 245 grain Barnes Spitfires over 120 grains GOEX FFG and got a five inch 100 yard group with an average velocity of 1576 fps. Looked to have possibilities with a little load experimentation. Probably should have tried 90 grains powder instead of 120. But, as I've said, saving the real stuff for the flinter.