Define horrible.... as that encompasses a lot of symptoms.
If it's rough or gritty, the same medicine applies to M&P's and Glocks, smooth out the trigger bar "ear" that depresses the striker block plunger. Not only does that get rid of the rough feeling that comes with the tool marks dragging over the edge of the striker plunger, it reduces the amount of effort needed in the trigger pull, not having to over come the resistance the roughness added.
I prefer the Glock style trigger over the 2 piece articulated one on an M&P, since I'm used to that style from having a G22. I replaced the trigger on my full size M&P 9 with the Apex 100-025 trigger, couldn't be happier.
My M&P 9 is of recent manufacture, so it has a much better trigger than the earlier ones... it's my understanding that it is attributed to S&W going to the Performance Center sear in the regular production guns.
I'd do the smoothing first and replace the trigger, then go from there. I bought my M&P 9 after my M&P 45 and did not feel that it needed the action kit like the M&P 45 did.
In addition to the trigger bar smoothing and Apex trigger, I also installed the Apex DCAEK (Duty Carry Action Enhancement Kit) and could be happier with it.