The D spring install is a MUST. It lightens the double-action pull by at least a few pounds. While doing this mod, if you notice the mainspring cap/lanyard loop pin is a roll pin, replace it with the "dog bone" steel pin. But I believe US-made Berettas (like the M9) come with the dog bone. While doing the D spring mod, you can also install the Elite II skeletonized hammer (mostly for looks).
For slightly thinner grips with much better traction, look into the checkered Alumagrips.
In the early 2000's, Beretta began putting plastic or plastic-coated parts in the 92 series. Current-production 92FS/M9s come from the factory with the following plastic or plastic coated parts:
• Guide rod (plastic)
• Trigger (thin spring steel coated in plastic)
• Left side safety (plastic or plastic coated, I'm not sure)
• Magazine release (plastic or plastic coated, I'm not sure)
• Mainspring cap/lanyard loop (plastic)
And of course, the stock grips have always been plastic, and magazine bases switched from aluminum to plastic at least 10-15 years ago, due to brittle aluminum bases cracking when dropped on hard surfaces.
I prefer all metal parts, so I switched all of mine out. Before I did, I weighed each piece on a 10-lb. digital postage scale, to compare weights of plastic vs. metal. Here are the numbers I came up with, if you're interested:
Guide rod - plastic: 0.1 oz., steel: 1.1 oz.
Trigger - plastic coated: 0.2 oz., steel: 0.5 oz.
Left side safety - plastic: 0.3 oz., steel: 0.5 oz.
Magazine release - plastic: 0.1 oz., steel: 0.2 oz.
Mainspring cap - plastic: 0.1 oz., aluminum: 0.2 oz.
So switching out these parts for their original metal versions adds about 1.7 oz. to the gun. In addition, the Alumagrips weigh 2.0 oz., compared to just 1.1 oz. for the stock plastic grips. So my gun now weighs almost 3 oz. more than it did the day I bought it. Since mine is not a carry gun, I like the additional heft and the reduced felt-recoil. I also just like the idea of metal parts in a metal gun. The steel trigger feels better and smoother than the plastic coated one somehow. Also, the metal safety fits slightly closer to the slide and has a crisper feel than the plastic one.
As others have said, use factory Beretta magazines (they come in 10, 15, and 17 round versions) or Mec-Gar (18 round flush fit, 20 round extended), and the gun will eat any and all types of 9mm ammo you could ever possibly source.
Enjoy!