The largest current .45acp mag that I'm aware of for the 320 is 10 rounds, though if Sig decides to put out mags for competition that have more...who knows. Right now Springer Precision has
extended mag baseplates but they cost as much as a whole mag from Top Gun Supply...
I've had and seen too many issues with > 8 round 1911 mags to ever consider one for serious duty use, but YMMV. I personally don't find 10+1 of .45acp lacking in any way...9mm, I would, but 45 not so much. I have a USP in .45 that has 12+1 and I honestly can't really think of any other reliable mainstream .45acp pistol that really has more than that? I'm sure there are a few (how much do 2011s hold?) but how many of them are in the sub-$700 market, like this is?
USPs are fine guns, but I find them...unergonomic to hold and shoot, compared to the 320. I love the USP mag release, but everything else about it just doesn't fit me well. That's one of the reasons I'm currently selling my HK45c...I love it, it's great to carry, but after years of struggling to shoot it as well as my 1911s or almost even my Glocks, I had to admit it just didn't work for me, which is a shame...but oh well. I find myself carrying my 320 in 9mm more and more, and shooting it really well even when cold (that is, no practice for weeks and shooting a FAST out of the blue with a great time).