Not a waterfowl hunter so this may be super dumb. When shooting magnum loads, is there a big advantage in any of the aftermarket chokes vs factory chokes. In this case the factory chokes are the ones that came with a Beretta A400. Situation will be a crane hunt with some ducks and geese possibly thrown in.

Not really.

Just put your modified in and have at it.

I just test fired a Remington 1187 and a Remington 870 with #2 steel shot. Both tested significantly better with a mid range Carlson’s Cremator extended choke tube than they did with their original modified Rem-chokes.

Were the chokes the same constriction? There are times when aftermarket chokes can/will be better, it’s up to the shooter if it’s worth while.

I have patterned some browning invector DS chokes vs carlson's waterfowl. Carlson's seemed to handle the steel shot much more consistently. Can't speak for anything else.

I used patternmaster chokes with 2 steel in a couple of beretta shotguns for a while. Patterns were tight and dense. I quit using them and went back to the factory modified. No difference in the shells to bird ratio. With the patternmaster the birds were shot all to hell. I like to eat them and didn’t care for that.

I had more fliers and a donut hole using the modified Rem Chokes at 30 yds.
The patterns with the Carlson’s were more consistent. No fliers and no holes in the pattern. The spread except for the fliers was about the same.

I had a shotgun with the Invector DS chokes and could not stand the chokes. I was convinced I had forgotten how to shoot due to the number of birds that were crippleds. Bought a Patternmaster for it and saw an immediate difference... and I do not pass shoot birds, I truly believe the choke patterned that poorly. Never had an issue with any other factory provided chokes the way I did with the Invector DS.

I wouldn’t say so either; Aftermarket Turkey Chokes on the other hand, do improve shot pattern as needed.

I'm no expert, but I know quite a few that think they are. My understanding after reading up on it is this. Steel is very hard so it doesn't compress well when choked like lead does. It can be over-choked causing the pattern to push back into itself.

So, if the aftermarket choke is longer, meaning the shot is compressed over a longer stretch, it should in theory prevent it from doing that and result in a better pattern. In what I've seen with my Carlson's steel shot chokes, which extend a ways out the barrel (while factor chokes do not), that appears to be the case.

I just tested my new BPS 20 ga. using 3” Black Cloud #2 steel shot. I tested both the Browning modified Invector Plus and Carlsons Cremator vented mid-range choke tubes. The Carlson tube produced a denser pattern at 30 yds. with no fliers. I also had no issues with wad hangups from the Carlson vented choke tube.
The Browning Invector plus tube produced a less dense pattern with several fliers.