+1 on night sights, and on the GAP floorplate, but I'd get one (they actually come in pairs) and install it to begin with.
If you want more than 10 rounds in a spare mag just get a G19 or G17 mag (15 or 17, respectively.) If you get in enough trouble to need more than your original 11 rounds you won't be worrying about the feel of the grip for your pinkie finger, although if you absolutely have to have a grip-size extension I think you can get sleeves for the longer mags that fill in the space between floorplate and the bottom of the mag well. Personally, I have more difficulty concealing a spare magazine than the gun itself, so I don't usually carry a spare. If 11 won't do it . . .
As for holsters, if you intend to carry IWB you will almost certainly be delighted with either an MTAC or SuperTuck, both of which are discussed and compared extensively on almost all handgun forums, including this one. (I have the MTAC, but largely because I had a CTAC before it and liked the belt clip and adjustment system.)