I cant say in the MK because mine is still NIB unfired but i do have two other 54s.
Personally i didnt buy 54s to shoot 45cal bullets. I can do that with a 50cal. IMO the 54 really shines with a 54x50 sabot or a big ole conical. Knight even packaged some of the MK 54cals with MMP purple 54x50 sabots and 325gr GoldDot HPs.
In my LK-93 with a 209 conversion i shoot 100gr and the Harvester Scorpion Gold PTs 45-300gr with a MMP 54x45 sabot. I prefer the MMP 54x50 sabots and in those i like the Speer GoldDot 50A&E 300gr or 50-300gr FTX bullets.
A great affordable 50cal bullet is the Rainier 50-335gr HP from MidwayUSA. Under $30 per 100. They are cheap and made to Alexander Arms spec for their 50 Beowulf ammo.
In my GMB54-120 i do shoot a Barnes 458-300gr SOCOM just because it shoots so darn well and is a perfect velocity match in that gun with 120gr of BH209 powder. It also shoots the 50-300gr FTX extremely well too and both performed flawlessly on deer this year.
Im not recommending you try BH209 but T7 LOOSE should get close to the same performance level and Knights max is 120gr of Pyro or real BP in the 54cal MK85s.
I would probably stick to 110gr of T7 or less in a MK
If you are not shooting much past 100 yards, 100gr of T7 and any 50-300gr in a MMP 54x50 sabot should do the trick. The 300gr GoldDot for 50A&E will be hard to beat on terminal performance at moderate velocity. The 50-300gr FTX should do ok but might fail to expand properly at longer ranges.....although i doubt it.
I would call MMP and get some 54x50 purples and some 54x45. Harvester 54x45 sabots are a bit loose in my LK93 54 but are really tough sabots and fit my GMB54 perfectly.
Both offer samples so you can find out which fits your gun the best.