There is a perception that heavies are OP, and to me it feels true that they are too strong. Some have called for nerfs against heavy/sentines suits, but I believe the issue is a complex one, and that heavies may quickly end up underpowered if they have their HP nerfed in addition to to fixing the multitude of other balance issues that need to be taken care of. Before any discussion of HP nerfs, several issues have to be handled.

The Issues:

The assaults are underpowered. Right now scouts can tank nearly as much HP as assaults while being faster, having a 2nd equipment, having a bonus to cloaks, being great a detection and scan evasion; assaults pretty much suck by comparison. Medium frame slots are very messed up in my opinion, think this stuff needs to be fixed anyway, so might as well use the slot fixes it as a means to buff assaults and other mediums. Others have suggested things like regeneration to give assaults a niche, and also increasing base HP of mediums. Anyway, one of the reason heavies seem to be so unstoppable is that the main combat forces (assaults) are so bad right now, if assaults were buffed, heavies might seem less OP by comparison; I think assaults will get buffed anyway, so its safer to just wait for the assault buff rather than just nerf heavies now, nerfing heavies followed by an assault buff could leave heavies comparatively underpowered. Too much change at once is dangerous.

The slow-down-when-hit effect on weapons. There is an effect that reduces your speed when you are hit by a weapon. No one wanted it, and no one likes it. It nullifies the mobility advantage of many suits, and it creates balance issues with many weapons; basically each round hitting slows down the target a bit, and high rate-of-fire weapons fire more rounds are fire more rounds at a given time interval, which makes the slow-down effect even higher. This gives the CR, aCR, HMG, and other high ROF weapons an extra (and unnecessary) advantage. This effect is the absolute worst with the HMG because of its extremely high rate of fire, this effect needs to be removed.

Projectile damage profile (-5% to shields/+10% to armor). Every other damage profile has the damage bonus to one kind of HP (shields or armor), and the damage deficit against the other type of HP add up to 0; for example, laser damage profile is +20% to shields, and -20% to armor, adds up to 0. The projectile damage bonus has a net 5% gain, this creates a slight balance issue for the HMG (and combat rifle).The damage profile for projectiles need to be changed to something with a net 0 gain like everything else.

The rifles and other weapons got nerfed in 1.8, but HMG was not nerfed. I think HMG's were at a good place in 1.7, but nerfing so many weapons while leaving the HMG alone gave the HMG an unnecessary comparative buff. The HMG might need a small 5-10% nerf after the removal of the slow-down effect, and the change to the projectile damage profile if heavies still appear to be overpowered after.

Heavies in vehicles. One of the balancing factors that heavies have is their low speed, but getting an LAV or any other vehicle allows a heavy to completely overcome the lack of speed. A popular tactic employed by heavies is the use of LAVs to close the distance between enemies, instantly jumping out and killing enemies, and instantly jumping back in the LAV to escape whenever they start taking significant damage. This wasn't much of an issue for me personally before 1.8 because my packed AV grenades were enough to destroy a militia LAV and its occupants, but now they can't because maximum carried grenades for all grenade types have been reduced. There needs to be enter/exit animations to give those around time to damage or escape the entering/exiting heavy, or at least something like a hack sequence to enter or exit. This 0 risk high reward tactic needs to be handled.

These things need to be out of the way first before nerfing the HP of heavies becomes an option.

Want to break the chains of 5 and 10 percent changes, how about 3% or 2%
If the changes are smaller my favorite suit wont go from "good in buildings or narrow passages" to "sucks everywhere" overnight -again.
At least gives us a chance to try something new before the suit is completely nerfed to worthlessness.