I use a Nordic Components spring and magazine follower in my 930. The spring is cut to 26 inches and functions perfectly. I didn't have any feeding issues with the factory spring and follower but they weren't up to my standards. Not smooth enough, hardly any resistance and that last round seemed like it would just ooze out of the magazine when I tested it. Turn your gun over, load one into the magazine, press the lifter up against the bolt and press the bolt release button. That will unload the magazine without chambering every round. That's a trick I learned from Mad Ogre on youtube for anyone who doesn't know.
Remember that 2 3/4 inches is the length AFTER it is fired and uncrimps. An unfired 2 3/4 inch shell is usually 2 1/4 inches long. Winchester buckshot, Fiocchi buckshot, everything I've tried by Remington and Federal target loads are all pretty much the same size and I can fit 7 of them in the magazine with no problems. 3 inch shells are about 2 1/2 inches unfired. Winchester 2 3/4 inch slugs (and Rio Royal as has been mentioned) are actually 2 1/2 inches unfired so they're like loading a 3 inch shell. I cannot fit 7 of them in the magazine but I do get really close for what it's worth.
The Nordic Components spring is a lot stronger than the factory spring so it doesn't need to be 30+ inches long to function properly. You have 18 3/4 inches of total magazine space in the 930 SPX. When the magazine is loaded with 7 rounds, you've got about 3 inches for the spring to compress. If you compress a spring like that for a long time (I did it with my 870 for 4 months), it will shrink slightly but still functions perfectly. The spring in my 870 went from 25 to 24 inches from four months straight of being loaded with 7 rounds of 2 3/4 inch Winchester buckshot. I didn't notice any difference in the the function of it, I had to take it out and measure it to find out it had been affected at all.
As for the magazine gap problems, try changing your follower. I've used the Nordic Components follower in my 870 and 930 for over a year now and all the problems I used to have have gone away. With the 930 specifically I noticed that my first round feeds smoother and most importantly, you won't get stuck on the joint between the extension and the magazine. The old design was grey but the new ones are red with 'made in usa' on it. That should solve the 'visibility' complaint, lot of people had.