I don't know if switching from 5 to 7 man or even 10 man, is going to improve the sport or encourage more people to play. But making it more FUN will.
Making the game more fun will require the vast majority of players to be honest with themselves about what and why they play, and require changes bigger than format itself. Until UK paintball works out that Sport Paintball is beyond the fiscal and ability reach of 98% of players the better the UK scene will be for it. X Ball is an elite, factory sponsored event only.
If 7 man is a step in this direction all well and good, but a dose of wake up and smell the coffee is also needed.
That, and a return to single paint sponsor events. In my opinion allowing novices access to trade paint is where UK paintball really went t*ts up. It cut a wedge between the sites and tournament, and led to every fat pie eating goober thinking he was going to be the next Skinny K; so "give me free bullets".