Yes, as others have stated, there have been continual ongoing revisions, refinements and manufacturing changes among the M&P's and their parts, going back to just about their official release in Jan '06.
From my first M&P pistol armorer class in '07, and throughout the subsequent recert classes (and calling back to the factory), I've learned of continual changes and product line rolling improvements within the model line.
It would make too long of a list to try and write down all the changes of which I've learned, but to answer the OP's question, yes, there have been some improvements made to the M&P 9's, including barrel revisions to resolve some erratically reported accuracy issues.
The last few recent production M&P 9's I've personally handled & fired, and have observed being fired, all exhibited very good inherent accuracy with some different loads.
The latest round of improvements commonly talked about when someone wants to discuss "triggers" has been called the "enhanced changes" in armorer classes. They include the slide stop lever assembly (yellow wire spring and the welded bumps on the inside of the lever); PC sear; and the 'H'=bar trigger bar assembly (better engagement with the sear using the H-bar trigger).
There's been a change in angle of the trigger bar loop (called by various names among internet enthusiasts and hobbyists). The 'S' bar has a 32 degree angle and the 'H' bar has a 40 degree angle.
FWIW, the new trigger bars are needed to help keep the trigger pull weight at normal spec, meaning not too light, and within the desired range for service/duty weapons (commonly expressed as being approx 5-6 lbs for service weapons). The Pro Series have a different/lower trigger pull weight spec, but they weren't designed as duty weapons.
There's a +/- of 2 lbs for the stock M&P pistols (and the .45's have approx a half a pound higher normal weight than the 9/.40/.357 models). Last I heard, the parts for the "enhanced changes" in the .45 models still haven't finished testing and approval for the .45's.
FWIW, there's been a small change to the sear housing blocks and newer frames, of a nature so that newest sear housing blocks won't fit into the older production frames. After learning of this in my last recert, I confirmed it by trying a new production block in a 2010 production 40c, and the new block didn't conform to the frame and fit in it. Things like this can be checked by the company (like for armorers), as their new computer parts system no longer lists general parts for M&P pistol models, but looks up a specific serial number to find the appropriate parts for a specific gun.
Also FWIW ... Yes, last I was told, LAPD has been issuing M&P 9's to new officers graduating their academy for going on 2 years, and they've reportedly been very, very pleased with them. (Still have a list of approved weapons that can be bought & used.) I was also told the LAPD shooting team has been very satisfied with their M&P's (and have been exploring some of their own refinements for dedicated competition guns
).
LASD has had enough time to try to wear out a number of T&E M&P's in recent years, and have also been very, very pleased with the guns. Hence, the huge order for them. I was also interested to learn that their huge order of M&P's included a small percentage of .40's, but the significant bulk of the guns are M&P 9's.
It's not been officially released, so it's not for me to identify the agency, but another major West Coast agency has been in the process of replacing their existing metal-framed compact .40's with M&P 40c's (after testing some various plastic-framed .40's on the market).
Shooting even the older M&P's can do a lot to smooth and lighten the "triggers". One of the other instructors has a M&P 9 from before the "enhancements", and while his trigger started at approx 7 lbs, after he'd been shooting & carrying it for a couple of years, the next time he checked it the trigger pull it was averaging close to 4 1/2 lbs, with stock parts.
I had my '08 production M&P 45 give me some initial out-of-the-box trigger averages of 8 1/2 - 9 lbs
(accuracy was still excellent, but I had to treat it almost like one of my DA/DAO revolvers). After I'd run a couple thousand rounds through it, I checked the pull weight again (as I realized it was feeling really light & smooth), and got an average of about 5 1/2 lbs (closely approximating the trigger pull of the same M&P 45 model that another instructor/armorer used, and in which he'd tried some aftermarket parts).
Of course, a bit later I got around to trying one of the then-newly revised striker assemblies, and a MA-compliant sear housing block (because it already had the large sear plunger & spring as part of the parts used to boost trigger pull weight for MA-complaint guns) ... and I found the trigger pull was back to about 7 1/2 lbs. I haven't checked it since then, but it's once again become smoother and seemingly lighter.
I don't dabble with aftermarket M&P parts because I only use mine for dedicated off-duty/retirement weapons (although I'll be receiving a new issued M&P in the near future, since I'm still working as a reserve as part of my instructor/armorer duties). I don't participate in competition, either. However, a recent production M&P 9 Pro 5" I tried on the range (another instructor's) had an
outstanding trigger.
I'm glad I didn't wind up with the 1/16" spring in the sear housing block I bought last year.
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