TaxPhd - I had an 11 round tube made for my Benelli M1 S90. The tube extends 9 inches from the muzzle of the 18 inch barrel. I can get 13 shells in the gun.
Locally, there are guys using 17 to 19 round tubes made for their 1100's. Are they practical for a IPSC match? It depends on the COF. I'm the match director at one of our local IPSC clubs and I recently started putting on 3-gun matches. I can't wait for one of these guys to come to my match and run around shooting through ports and around walls with it.
By the way, anyone around the Houston Texas area who would like to shoot the 3-gun match it's scheduled for this Sunday the 21st. Match information can be found at http://www.uspsa4.org/matches.htm
I will be using Limited and Open divisions. Magazine capacity will be restricted to 8 shells for Limited. So don't worry about having to compete with the super long tube guys. You can get by with a 5 round pump gun. Just have a way to carry a box of shells with you.