does anyone have any issues when they shoot 165gr as opposed to 180 gr ammo? I know that some guns, length of barrel, twist rate, etc shoot some weight bullets better than others. Was the M&P 40 full size or compact designed for a specific weight bullet?

Gun is MP40C. Having put through over 300 rounds of Gold dot 165g, I can say they consistantly shoot low (4") compared to 180g FMJ at 10 yds. 180 shoots POA with everything I've tried. Oddly enough, Extreme Shock 100g AFR's shoot POA or slightly high. I wouldn't think bullet shape would matter at these distances but who knows. One thing I have really noticed more than anything else about shooting is that target height makes a huge difference. If the the target is lower than my arm being level, I tend to shoot low due to sight picture and vice versa. Not enough to affect combat hits, but bullseye for sure. For example, last week someone left 2 shoot-n-c targets up with only two holes in it, so I figured I'd put them to good use. One was pasted 12, one at 6 to my natural point of aim. I shot high on the 12 and low on the 6.
During cease fire, I pasted my own and bingo, i was shooting dead on....wasn't the bullet weight that was affecting shots, but my POA relative to my eyes/arm.

I was informed by S&W that the 40 cal M&P's liked the 180's. My FS and compact are dead nuts with 180's. 155's are all over the place. Haven't tried anything else yet, but I'm getting around to it.

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load, shoot, repeat

My experiences with Glock 27,23,22, and S&W 610 revolver and 165 gr bullets have been positive.

They will shoot just about anything. The key is to shoot enough of what you carry so that you know where they will hit. If your gun shoots 4"low with 165's you will know going in, and can compensate by aiming a little high.