Two Problems...
Does the Height of the 870 Banded Front Sight match the height of the 590 Rear?
If not, you may not have the proper amount of adjustment in height to get shots on target and accurately. (That is if you can adjust their rear sight for height. I never had one in hand to use so I cannot tell you off hand.)
and
Does the Barrel Profile at the muzzle end of an 870 barrel, match that of a 590?
If not or even close enough, the front sight won't be attached well enough and may shift around, defeating it's purpose, shot after shot.
Best advice I can give you is, email XS and ask them if the 870 Banded Front Sight is compatible with the Mossberg 590 Rear Ghost Ring Sight. I have never had any problem contacting them and getting a good reply.
Best of Luck!
Rick