Canuck 223.,
I've been playing with two different guns for 3 gun and Action Shotgun. If you are following IPSC rules then an 8 round mag is the way to go if you want to remain competitive (and in Standard).
For IPSC 3 gun, I've been running an 870, 20" IC barrel with Scattergun Ghost Ring sights, six round side saddle, +3 Choate or Scattergun (Wilson combat) mag tube and a Speedfeed pistol grip stock (full stock, C7 Styling). This 870 is one of my LE teaching guns so it has had alot of rounds through it.
In the states I run an 1100 with +5 round tube, 22" IC barrel, Scattergun ghost rings, 6 round side saddle and a heat shield. I find the heat shield is a must if you're doing alot of firing.
Last year I started experimenting with my Benelli Nova with a Choate mag tube so that it now holds 8 in the tube. It has a fibre optic front sight the same as Tom Knapp uses and I've found it to be very quick and pointable and the pump action feels quicker than my 870 (against the timer, I've only noticed a slight difference with the Benelli being faster).
When I was shooting action shotgun at Barrie a few years ago, I was running the aforementioned 870 with an 11 round tube, 24" barrel with heat shield and screw in chokes (usually IC or Modified). It was very quick and alot of fun.
You're right about the 14" barrel being a bit of a limiting factor as far as mounting a tube under it. It'll just look weird with a long tube on it. Why not just find another barrel in 20" or 22"? With the 14 and +1 tube you'll have to reload the gun more often which can be a pain. That being said, as Repete says, "As long as you're having fun!"
