The barrel extension from XTX is nothing to do with shrouds etc. It is exactly the same diameter as the barrel, look on his website, I have one on my Scorpion T10 looks ok and allows me to fit a normal silencer, but as I recently put a longer air cylinder on my Goldstar SE I bought a Parker Hale silencer, they are the only slim one I know of and at £24.99 I'm sure it will be much cheaper than considering changing the barrel.
As far as shot count goes I cant answer that as my Scorpion is Tench regged and my Goldstar SE is BSA regged but if your current cylinder is as I suspect 250mm long and you buy the 325mm cylinder you should get 37% more shots from it. Best deal on air cylinders I found was Airtech.
John