I too am an experienced competition shooter (NSSA and NSCA ... just got back from the NSCA US Open yesterday) and am very familiar with these guns. Here is my take:
1. The Benelli is a proven system that works great. They are more targeted for Home Defense and Hunting than the
target sports but some are being used ... effectively. The inertia system will kick a little more than a gas system.
2. The Beretta 391 is the current gold standard for autos in competition. Most work well (some need tweaking) and they are very easy to point and swing for most people. Recoil is light and aftermarket parts and accessories are readily available. I have 3 of these and they all run perfectly (I also have a Benelli). The 391 would be my first choice. The A400 is too new for me to have an opinion on. It is very light!
3. I know several people shooting the SX3 locally and they all work well. I don't have any personal experience but these appear to be a good value.
4. Historically, the Brownings have been less reliable than the Berettas. The Maxus is fairly new and is a nice looking gun. One of my best friends just bought one (he is a big Browning guy) and he loves it. I shot it and it was very similar to the Beretta 391 IMO. He had a couple of FTFs when the gun was new but after that it worked well. I am not sure how much was changed mechanically from the Gold model but broken firing pins and FP springs have always been a big issue with the Browning autos.
Don't rule out the Remington 1100 either. Nothing shoots lighter (recoil wise) and they are proven performers.
The gas guns will need cleaning more frequently than an inertia gun (Benelli) but it is fairly easy and only takes 10 minutes after every 500 rounds or so (more often with the Remington). I have run as many as 2500 rounds through a 391 without cleaning with no issues.
My vote is for the Beretta 391!
Hope this helps.