First of all I do not have one of these followers so this is just my opinion from reading this thread. The SBE Precision follower does not look like it will change the number of rounds in your mag tube. It just replaces the 930 factory follower with a sexier one. Purchase one and check it out in the 930. Also try and install it in the 590 and see if it works. If it does buy another. I purchased a number of aftermarket mag followers from MCARBO. The followers worked great in the 500 series pump guns but not that great in my 930 shotguns.
Thanks, I went ahead and got one from SBE for my 930 as you suggested. I'll let you know how it goes.