First off, Lee molds are made to a price point. I suggest you ask on the North South Skirmish Association board for someone with a quality used mold in the proper diameter. Buy once, cry once.
I have the Lee "Improved" minie mold and it won't shoot in any musket I own and two of them are 2moa guns. I don't know what they did to "improve" it but it didn't work for me in 6 muskets and it's not like I didn't try. The ONLY Lee minie that any of my guns will shoot well is the flat nose "target" minie but they've discontinued it in 58cal so to get one, it's ebay. I have 3 of these molds as these bullets don't tolerate sizing much and they are for those of my guns that require a 575 minie. That minie out of those guns will shoot one hole groups at 50yd. I also have the Rapine version of it and it's also a tack driver in my 580 size muskets. So I really like that bullet design.
Beagling molds with tape sorta works. The minies will not be round when they come out. I experimented with it and it will help a problem gun but will not yield consistent target grade results.
Using a sacrificial bullet and valve grinding compound does work, but you risk trashing your mold if you aren't careful. Now you're into the situation where a good quality used mold is the same cost. It works but be careful.
Powder Coating- I've experimented with it in minies with mixed results. What I did observe, it would "bump" up the dia about 2thou so minies that absolutely wouldn't shoot well shot better but not to target work standard. So in one sense, it works, but in another, it's not a cure for undersized minies.