These are the changes I’ve made to 2011s I’ve had. They were improvements for me; some may find them dumb, which is also fine.
1. Mag release spring - either a lighter spring or trim the existing one. These springs are often too stiff and I don’t drop the mag. This may possibly be due to my left-handedness - I use my trigger finger to hit the mag release and I may not have as much leverage as you right-handed thumbdumpers.
2. Short trigger - I don’t have small hands, but I tend to use the tip of my finger and a short trigger helps mitigate this bad habit that I’ve never shaken.
3. Steel mainspring housing - I’ve added a steel MSH to add an ounce or so of weight in the heel (when the grip/MSH is polymer). Maybe splitting hairs, but I like the feel of it better.
4. Reduced power recoil spring - some factory guns are badly oversprung. As a competitive shooter, I often hear the misconception about “powder puff” ammo that non-competitive shooters assume we use. A lighter spring changes the recoil impulse, but has little to do with light ammo (at least for me). My gamer ammo is slightly spicier than what I usually carry.
5. Aftermarket fire control parts - only worthwhile if A. your gun has MIM junk (sometimes MIM stuff is ok), B. you want to improve/lighten the trigger, and C. you will shoot it a lot (10,000+ rounds). Some MIM parts will hold up, but some high volume shooters go ahead and ditch them when doing a trigger job.
Having said all that, and the Mac 9 being your first 2011, I would just shoot the hell out of it and not do anything to it for a while, if ever.