XD Depends. HVA requires a massive buff, though. Quite a few weapons in the game receive little to absolutely no benefits from equipping it.
No one knows, but it's definitely the most controversial weapon attachment: the downside is such that in many cases the velocity bonus is not worth. In fact many weapons perform better without HVA.
I feel MCG should have different mechanics. Pre-fire spin-up and consequently faster rate of fire would make it more minigun-ish. It's quite sad that it doesn't even have the highest infantry weapon RPM in the game. All those nice spinning barrels for nothing.
At least it gets a bonus velocity, unlike the Gauss Rifle Burst that gets no improvement on velocity at all.