I have 2 18" SPXes, with OR3Gun components, including the spacer tubes. Just under 1000 logged rounds, the majority light target, but both right at 250 00 and slug, no FTF/FTE/light strikes, etc., on either one.
When you say you have rust on your tube- I'm going to say that's a big part of your problem. Once you have corrosion and/or carbon build up, you'll start having problems. The JM Pro NiBO gas piston is a good investment, as is the JM Pro NiBO mag tube (unless you get a surplus stainless tube, as I mentioned elsewhere). If you have the blued gas piston, and if you have rust or carbon in the rings, they won't be able to move and flex to get a good seal. Add gunk around the tube, and things get worse.
Hot blue is very different from swab-on blue. Unless you tanked your mag tube, the bluing won't be deep enough to give real protection, hence the rust you see. The spacer tube allows blow-off of moisture, using the excess gas. On a coated, or deep blued tube, that works well. On bare steel, or a "shallow" blued tube, it will cause rapid oxidation, as some of the moisture is rapidly dried on the tube, which in turn adds to build up and drag.
Please don't get me wrong, I really like the 930SPX, but Mossberg made it an entry level tac gun. The shorter barrel creates some issues that the bird & field guns won't have, and it's not like any major departments have adopted them and run tens of thousands of rounds. 3 gunners have,
but they generally make some mods that you and I probably wouldn't, and even then, without a corporate push to make OEM changes, by this time the base platform is a bit lacking. I like the variable gas system. I like the relative availability of parts. I like the relative low cost of spare parts. A Stoeger 3000 would be an alternative, but can't accommodate the range of loads the 930 can, so...
OR3Gun is a better tech source than Mossberg Customer Service (God bless 'em), so their advice is good.