If the REAL fits well, it will shoot well. If it fits loose, then Zimmer's description would apply to any bullet.
If heat from the burning powder can pass up the base of any cast lead bullet, what he describes will happen. Hot gasses will melt the lead and blow it past the bullet where it can plate onto the bore with combustion byproducts. It can't be accurate if that happens.
If any bullet can engrave the rifling enough to avoid slipping, but not be too tight, then a gas seal can be made to fit over the powder to prevent hot gasses from searing the bullet. Wool, paper, grease cookies, plastic bases that look like cup seals from a car's brake cylinder....whatever.
REAL is a short bullet that should out perform a round ball in medium to slow twist rifles. If you can find the right seal, lube and load combination.
I'm mostly interested in 54 caliber, so I have Lee mini balls, T/C Maxi-Balls, Hornady Great plains, whatever I see at the black powder shop if it interests me.
I have Lee REAL molds for 50 and 58, but have not tried them yet. The 50 is the lighter of the 2 choices, it's for my Trapper pistol kit, which I haven't finished yet. The 58 is for my Cabelas Interarms Hawken, which has only been shot with Patched Round Balls so far...So sorry I can't give any feedback.
My point is that any cast bullet can suffer from gas cutting, not just the REAL, so get someone to send you a fistfull to try. Heck, it'll be fun and you might learn something.