The welded lifter on a 930 is one of those things that may not be necessary for you depending on what you land on for a loading port, loading method and how your thumb cooperates with it.
The old-school weak hand load from caddies requires that you keep your thumb in contact with the lifter for a smooth load. Sliding your thumb off at the end of the lifter makes smooth very difficult when you have to slide it back up the lifter to milk the next shell out of your hand and into the port. It usually gets caught between the lifter and receiver and takes a piece off of your thumb if you wanted it back in a hurry. On a stock Mossberg 930, you can't get a shell to click into the magazine with your thumb perpendicular to the base of the receiver because of the lip on the receiver that jets out past the follower. As a result, you have to run your thumb at an angle that has you sticking it part way into the magazine tube to click the shell in. The forked lifter doesn't let you have your thumb back without a price. For weak hand from caddies, you need a welded lifter and the front of the port opened up to where your thumb can click a shell past the shell stop when perpendicular. You also need the forearm overhang reduced enough that your index finger knuckle doesn't keep hitting it as you load.
For twins loading, your thumb isn't milking rounds into the loading port, you are sweeping them into the gun two at a time from a side angle and hoping to get your thumb back. For a gun being used for loading twins, having the loading port angled down so that your thumb naturally flows to the port is key. If you have the end of the port opened as you would for weak hand caddies you are most of the way there. The forearm will be even more in your way and you will still want your thumb back. What works even better for some than a welded lifter for twins is one that takes the lifter's factory 'u' and opens it into a wide 'V'. Your thumb doesn't need to slide back up the lifter with a twins load for the next shells as you have to go back to your belt for them. The easiest way to keep your thumb from getting caught in even a welded lifter (which still happens when your loading port is opened too far) is to not have the trap there at all to get caught in. Opening the lifter works better for some people than a welded one. You also gain the ability to fix a double feed from the bottom. The only down side to the wide 'V' is you have to have a port that smoothly accepts shells without requiring the lifter to serve as a ramp.
Quads can again introduce the issue of hanging your thumb up as you run your hand back up on the lifter with the second set of shells. This is less of an issue than with weak hand from caddies, especially since the second set of shells helps to hold the lifter against the bolt as you acquire the port again. The forearm is really in the way for quads on the 930. That second set of shells has to glide over your forearm or receiver while you deal with the first and the flatter the forearm for that distance the better. The 930 is thought to be a really lousy platform due to the height of the forearm off of the receiver. Truth is, the magazine tube sits shallower in the loading port than a lot of other autos, so the loading port itself doesn't have to be as aggressive or deep. That doesn't keep people from cutting them like other guns, but that's a different topic. The magazine tube on a 930 simply isn't buried as deep into the receiver as other guns. Unfortunately, the gas system parts had to go somewhere and they force the forearm to protrude from the receiver more than other guns. Since you have our forearm retainer you are half way there. The rest is getting that forearm on a serious diet if you plan to quad load with it. This can be done by hand or you can send it to us.
Your existing port will probably work for quad loading as is. You might find running a bit deeper on the sides at front to be worth doing as you progress. The point of greatest frustration when learning will be the amount of forearm you have hanging in the way.
