The thumb safety isn't connected to the trigger in a manner which would affect the trigger pull. The assembly blocks the rearward movement of the trigger when it's in the On-Safe position.
My personal M&P 45 had a trigger on the heavy end of the possible range (7 1/2 lbs +/- 2 lbs versus the 9/.40/.357 models where the trigger is expected to run in a spec range of 6 1/2 lbs +/- 2 lbs) ... which meant mine was averaging over 9 lbs at first when averaged on my digital scale. It was stiff and had some roughness in it, but the gun was amazingly accurate despite the trigger. I just had to work at my trigger control/press.
It became much smoother with use and it became even easier to experience the amazing accuracy of the gun. I enjoyed shooting it more than my 1911's and my TDA .45 Smith's.
While I haven't sent mine in for a Performance Center trigger job, and likely won't, I've heard they do a good job. I haven't had the opportunity to try one of the Performance Center sears, either.
My M&P 40c had a somewhat better trigger out of the box than my M&P 45, although still with some early roughness which disappeared more quickly than it did in my M&P 45.
I recently replaced the original MIM striker in my M&P 45 with a new revised version, which is thicker in some dimensions and appears to be a stainless (cast?) part instead of MIM.
When I took it to the range I was expecting a certain amount of 'new stiffness', if only from the new striker spring and the new, unworn surface of the striker's foot engaging the sear. Instead, it felt like it was even smoother than when the previous well-worn striker had been in the gun. Nice.