The 50 150 rule wouldn't make it unplayable, but would certainly fuck with balance.
First off, combat armor would be a little overpowered as you effectively raise the resistance of it, and its already kinda powerful as is, to the point it effectively negates small caliber shots to the front 90% of the time, and reducing max incoming damage as well, putting more weapons in the "harmless" threshold. Shotguns especially would be really nerfed, and melee enemies too.
The armor was pretty well balanced in this, and I'd estimate that 50 150 would be far easier up till facing sectopods.
I don't want to speak for scorch, but I doubt hunter killers would apply within the first 2 years for sure, possibly even 3 years since migs are a terrible match for ufos. I'd be utterly shocked if he added hunter killers then, and if he did, it would probably be an extremely rare 1 off kick in the nuts.
Private car has an upgrade to hummer at promotion 2. 5 seats, almost as fast as private car, and of course infinite range.
Anyways on to a bit of my own input.
Not sure when it happened, but allowing events to time out as you travel to them is extremely nice : i actually have a reason to use private car now, instead of vans at all times, since madman killers in particular require speed.
Exalt brainers seem too psionically tough, anyone with bravery under 40 is putty for them, and I even had a bravery 70 guy get mind controlled despite high moral. I can understand A brainer being a threat, but and having psionics, but I'd notch down its power.
Considering you altered drones recently, any chance we can get alternative drones for scouts with smg, or rifle ammo? Particularly a higher capacity, the 50 damage burst is great, but 6 shots really lowers flexibility, especially since it cannot carry a reload by itself.