Well here's my view;
I was recently offered the loan of a standard XB12R muffler to try on my 06 XB9SX. With no servo or cables, I rigged a wire and hook to the exhaust valve so that I could set it either open or closed from the side of the bike. Adjustment would not be recommended whilst on the move unless you are a veteran Cossack stunt rider! 
Starting with valve closed, I have to say that the XB12 unit is very good on my 9. So good I wonder why they didn’t fit it to the 9 as standard after introducing the XB12? Perhaps they had a shedload of 9 mufflers to use up.
From memory, the standard 9 muffler seemed more constricted in that the smaller bore internals gave good low end from tick over, but on my City X it was always indifferent in the mid-range even with a K&N and white wire mod! The 12 muffler on the other hand gives fractionally less snap from tick-over, but once the gases start flowing it delivers a much nicer spread of power all the way through. Holding it in mid gears is more fun. Roll on in top is still good and impressive from 80 upwards. One way to describe it is that it feels more like a shorter stroke 12 than a strangled 9!
One of the best features is the noise, with a deeper more soulful bass note, and the bark at higher speed roll-ons is something to savour. 
For my own riding style, I don’t think the 9 benefits much from the exhaust valve, and I have no wish to try it flat out anyway. Likewise, the slightly softer low end probably reduces stress on the gearbox and belt. I think that the 12 muffler makes my 9 feel less stressed generally, although city slickers may prefer the standard 9 muffler in traffic. 
I now have my own low mileage XB12 muffler to run on the XB9……….but it is heavy compared to the Stealth! 
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