Hi
Steel is not recommended tighter than modified chokes and also not recommended for older sxs shotguns. But on the other hand i have used small size (number 4 and smaller) steelshots on my baikal sxs without problems.
Full choke cuold do quite bad patterns with steel so try it first.
There was test in one hunting magazine some years ago where they shoot over a thousand steelshot round through older shotguns. There were some slight changes on chokes but they can't find any significant changes on patterns and all three shotgun survived.