I really enjoy the feel of my M&P9 Performance Center's trigger. However, I really hate the look the trigger and prefer flat faced triggers. I really like the Apex flat trigger. Would I be able to use and Apex flat trigger, but keep all the performance center trigger upgrades or would I have to get the whole Apex kit for it to function properly?

Why not just use the Apex kit and adjust the springs to preference? Function is not an issue in either case. I love my flat trigger Apex kit more than my PC setup.

To answer your question, yes, you can just get the trigger and keep everything else stock. But, you can also buy the full kit and experiment with the different trigger pull weights to see what works for you. That sear that Apex provides is amazing - I've been told it is an improvement even over the Performance Center version. I have Apex'd my Shield and most recently my 9c and love both setups.

I also hate the M&P "curved" trigger. I remember one member took a small file and lightly removed much of the lower "hook" on the trigger. I haven't tried it, but it might be my first option, instead of spending money on a new trigger.

nice - a good example of this would be the trigger on the FNS 9, which I think was the look he was going for.