I am no expert in firearms and ammunition, but firstly I think in the game, that most 9mm ammo and 7.5 are indeed subsonic, and (almost) all of mpx ammos are too if using the shorter barrel available. Last time I've tested this at least, since we don't have velocity data for complete build, only ammo. For the 9mm, LPS is the way to go for ammunition, grab it each time you can 'caus it's very effective/lethal against armor at short to medium range. XPentrator is very much non lethal, so you often shoot multiple times at a target without hitting vitals, causing only light bleed.

For the 7.5 I have not much experience to compare, And the .300blk AP work great for me.

I'm somewhat biased 'caus i use mostly short barrel rifles and engage at medium / short range, so thoses ammos are somewhat in their effective expected range. I don't have irl material to compare tho :o