RE: Full choke with steel in Rem 870?
Many shotguns will explicitly say in the user's manual that steel shot shouldn't be shot through full choke. For the reason you alluded to. Steel doesn't compress like lead putting greater pressure at the point where full choke compresses the pattern.
Many would say full choke isn't needed for the ranges you typically shot with steel anyway. From personal experience I'd agree. For shooting over decoys improved cylinder or more likely for all around waterfowl, I find modified give good results for patterning with steel. I think there is even an improved modified choke that's between modified and IC.
You'll hear people say they shoot steel through full choke from time to time and "nothing bad happens". Well that may well be true. However, if the manufacturer says for their shotguns/models "don't shoot steel through full choke..." of similar warnings, it's best that you follow their warnings. Doing this or anything explicitly not recommended with firearms is dangerous. It may be that over time shooting steel through full choke will stress the end of the barrel to the point where it will without warning fail.