It's important to define "Low Tier"
Keep in mind that many of the less commonly used weapons (Like the SKS and M16A2 /A4) are also incredibly inexpensive, and still 2HK. Also it's worth note that the Mosin Nagant and M24 already have AP ammunition by default.
It's worth pointing out that in INS2, AP ammunition was a destructive must-have addition to the meta that completely negated armor to such a degree that not using it was effectively gimping oneself. Even free pistols like the makarov could effectively be turned into primary weapons due to the hyper-lethality of AP rounds. In reality, even the most expensive 9x18 rounds aren't punching through a steel or ceramic trauma plate rated for rifle calibers.
As far as shots to kill, many of the weapons currently used are 2 to 1 hit kills, with exception to pistols, SMGs, and some variance as far as shotgun spread goes. Enemies in armor don't really change shots to kill either, with light armor usually only buying a few extra pistol rounds, and heavy armor only buying a player one extra shot against a FAL or EBR, and that's coming at a pretty considerable hit to mobility and three points that could be put elsewhere.
If nothing else, I'd personally keep shots to kill the same for all the armor except no-armor. All weapons would have STK reduced by 1 unless already at 1. This would both make high mobility play even more hardcore and dangerous, while adding a marginal amount of viability towards armor so that it isn't viewed as the handicap it currently is. It would also make inexpensive weapons like the AKM or M16s even more viable without having to fundamentally change their handling, or adding a must-have item that overrides a weapon's damage model and negates armor altogether.
@Goat-Walrus well who cares that it was a bad meta, it was so much fun, with the fast time to kill, the small maps, the kind of dark and unforgiving atmosphere.
To be honest the devs should just take a big and long look at Insurgency source and change Sandstorm to be more like the old Insurgency.
@Zayzek The entire point of @Goat-Walrus response was that it's possible to get back the feel you're missing from previous games without bringing back the stupid meta of AP rounds. It's possible to make guns more lethal without requiring everyone to spend 2-3 supply points every game to get that experience.
It's also unrealistic. Pistol caliber AP rounds simply are not effective. The weapons are weaker because the cartridge is weaker, there's no getting around that without bending reality. Luckily these weapons tend to make up for low damage with high rates of fire and good controlability.
I agree with @Goat-Walrus.
In Insurgency 2014 everybody used AP ammo because both standard and HP ammo weren't viable, which is simply bad game design. Restricting AP ammo to certain weapon types is actually one of the better decisions that was made in Sandstorm.
Making low caliber rifles one-shot enemies without armor would be a good way to balance things out, since both these rifles and light armor are pretty underused. There isn't really much use for equipping light armor now because it only helps against pistols and SMG's, which are both quite rare. However, if light armor would let you take an extra hit from low caliber rifles, it would become a more viable option.