Holy crap there's a lot a lot of flat out wrong info in this thread.
YES, the heatshield can be put on a 590A1, with a little work.
Added weight? Please. The shotgun carries 9 rounds, not including the sidesaddle or speedfeed stock. Try an all-steel 870P with similar setup if you think the 590A1 with guard is heavy.
Take a shotgun class in west Texas (or just about any part of Texas) in the summer. Heat might become an issue.
The 590 has a lug as well.
The 590A1 does NOT have "the bullshit LEO trigger."
In 7 years, I've never had rust develop under the guard. Get some spray CLP and blow it under the guard. Even if you don't, it is not that big a deal to remove it on a quarterly basis as opposed to the 10 year PMCS cycle which I wouldn't advise for ANY weapon short of ye olde pointed sticke..