The Burwell trigger job shown in the link
http://www.burwellguns.com/misc/M&Ptriggerjob.pdf is probably over 10 years old now! (M&Ps came out around 2005, if I remember correctly). After the first couple of years, and the Burwell job, and then Apex getting into the market, S&W started paying attention and has progressively improved the geometry of their factory sears. I think it is to the point where it is counterproductive to do much with it.
Early M&Ps had very little trigger "reset" - which is often desired by competition shooters. Somewhere along the line Apex designed a part that pushed the trigger bar to the side and created a sense of "reset." I don't know if they even sell it anymore, because the M&Ps have had a built in "reset" feeling for quite a few years now, based on factory "in process" improvements in design of the trigger bar geometry.
Smoothness is another issue -- there the Apex trigger would, I think, be pretty neat -- to deal with the long take up and the overtravel. Worth $80? Maybe. Added: my new Shield arrived, and we're on the way to the range tonight. The trigger is just fine the way it is; I don't want it any lighter than stock!