Nice guns; I just sold one a couple of months ago. Power of mine when I bought it secondhand was about 2.2 ft.lbs. I sent it off to be modded* and it came back doing 4.8ft.lbs which was perfect. If it's standard I wouldn't expect any more power than 3 ft.lbs. The next model up (Sport) does a bit more - 4 to 4.5 as standard.
I shot Exacts through mine mostly as I used it for pistol FT. It one-holed with those at 15 yds, but it was the same with Mozzies and AA field. It's the usual advice of try a few decent brands and see what it likes.
In theory the single-shot should be more accurate than the mags but I never noticed a difference in mine.
An 11mm scope rail is available as an optional extra, as is a weaver rail. The 11mm is steel but the weaver is plastic so go for the 11mm if you're mounting optics or red dots.
*Phil Hassell did mine but I don't think he's doing them any more and by all accounts is now very difficult to contact. A competent gunsmith should have no trouble though.
“We are too much accustomed to attribute to a single cause that which is the product of several, and the majority of our controversies come from that.” - Marcus Aurelius