IMHO sometimes we make too big a deal over this. Most MLs, while quite accurate, aren't super duper tack drivers. I have a very similar gun to yours, a T/C Hawken re-barrelled with a Green Mtn .54. It is for round ball (1:70" twist).
Get yourself some .530s and shoot with .010-.015" lubed patch. Also get some .520s (hornady has these) and .020" patches. See which one you like best. My personal choice is the .520 rb w/.020" patch as it loads easier. Accuracy between the two is very comparable and quite good either way. I've never bore-slugged a rb barrel and been shootin' MLs for >35 yrs. Just go out and have fun...of course, if bore-slugging your barrel meets your definition of fun, by all means you should do that.
