Many (if not most and nearly all that I have seen) 2011 shooters modify their followers and or slide stops to prevent the slide from being locked back on an empty mag. This does two things: 1 prevent the possible overinsertion of a magazine when reloading to a locked back slide and 2 prevents premature locking back with a round left in the mag.
If you have older SVI tubes, you are in a position many would like to be. In an SVi tube, I prefer Bolen springs, followers and basepads. So long as your feed lips are properly set, you can easily get 20-21 in the mags. One major issue with STI tubes and the Bolen innards in that they go deep enough into the tube when loaded to get below the indentations and the follower rolls over creating a big ole rattle. This is not an issue with SVI tubes as they have a different style of indentation which extends lower into the mag body.
All of that said, many people prefer Grams internals (followers and springs) as they believe they are easier to setup and more reliable.
My .02? Call Mike or Barry at Shooters Connection and they will get you what you need.
Oh, and get an EGW gauge while you are at it so you dont go over on length!