I have the "Big Dot" on an M&P40C, and "Small Dot" on a 1911 and one other gun (S&W CS45).
Not worth a lot (especially the big dot!) for paper punching, but little sacrifice for SD.
The front sight on the M&P was a bear to install - no issues with the rear sight, and I knew about the spring

. You really can't use a pusher, and have to plan on a LOT of filing, etc., for the front. I filed the heck out of things, and finally decided that a little careful bashing (with a nylon punch) was in order. Went right in.... But it took hours.... I've done a few such swaps (that 1911, for example), and have a good pusher, but this was a bit different. That said, a good file or two (file the sight, not the slide!), and some fine metal-cutting "sandpaper", a mallet and a plastic pusher (comes with the sight), and you should be able to do it. Or pay somebody under $50....
From what I see here, the 40C and the Shield slides are essentially identical in this respect, although the front sights probably are different enough that XS has a different model for the Shield.
One thing I did find is that the small dots are
acceptable for paper punching if you're not really serious about it. The front sight is only slightly larger than "stock", and the rear sight is more of a hindrance on vertical placement than anything else. It also seems that I can switch between the XS and "standard" sights (different guns) without much thinking. Just seems to work.
Just IMHO (old, and semi-retired rent-a-cop, among other things), Baccusboy's source is probably accurate. Particularly the paper plate. (Move the plate around - vary the height.) And note the "sitting down" part - one argument for ankle rigs!
IMHO, SD means putting six rounds or so into a 6" circle at 30' and under. (Practice at 30' - makes closer-up shooting seem much easier.) No extra points for 100 yard shots with a snubbie, and, here in Ohio, probably a good way to go to jail even if it's a righteous shoot!
Regards,