Awesome write up. I had the M&P 40 for a while, but I don't have any .40 guns any longer. If I were going to get one again it will probably be the M&P or the USP.
I had the M&P 9 Pro (4.25") for a while and really liked it. I sold it a few months back to pay for something else and have regretted it since. I think I'll pick up a standard M&P and add the Apex FSS kit to it. Maybe it makes sense to go with the 40. I've heard the M&P 40's convert to 9 quite reliably.
At any rate, I do currently have the M&P 45 midsize and I love it. I had the fullsize and compact in the past and sold both. I didn't like the look or feel of the long slide on the fullsize and I didn't like the grip on the compact. The midsize is perfection. It's very similar to the M&P 9/40 fullsize, it just has front serrations. I have the Apex DCAEK in mine and it's excellent. The factory trigger wasn't gritty or bad, it was just extremely heavy... like 9.5lbs. The Apex kit brought it down around 5lbs and even more crisp. It does have a bit of over-travel so I have ordered the Apex trigger to drop in which is supposed to reduce that. Once I have it installed and run it through it's paces, I'll post up a review on that one as well. It's really a shame the M&P 45 midsize seems pretty rare. It's very nice. It's good size for concealment and it's 10rd capacity doesn't both me because of it's size like the HK45 does. That gun could easily be a 12/13rd gun with it's mile long grip.
I do wish Apex made the RAM for the 45 guns. The reset on this one is non-existent. It doesn't slow it down much because I've shot the P30 with LEM trigger quite a bit. It's not a big deal though - to me.
My Gen4 G21 is probably my favorite 45 overall, but for carry, the M&P 45 midsize is my favorite for a carry 45.