Yeah, I hear what you are saying but I've never found that too unbalancing. It is also the RAW for the gas vents so I guess play testing didn't find it so either (Don't tell me they didn't play test this. You'd shatter all my illusions

) I find it difficult to get enough recoil comp on most weapons even doing it this way.
Now for the the question of balance on mini-guns. I think it would be fair. First, the mod requires 2 mod slots so is kind of expensive. Removing the gas vent system from the Ares-MP would not give you any slots back but I guess you could stick a gas vent accessory on it. It also requires a facility to install the high velocity mod and is a 30 threshold to achieve. Now, I'm not sure about the RAW on this, but we rule that those tool sizes are the minimum so you
need a facility. A 30 threshold, especially with decreasing dice pool rule in effect is very hard to hit. Even without it, it will take quite some time to achieve. So once you are done you have an Ares MP LMG that fires 15 rounds and may or may not have a gas-vent 3 on it but you will have also used up 2 of your 6 mod slots. It will have cost you an additional 3000¥ as well. So 1500 for the gun, 3000 for the mod and 400 for the gas vent (or 2400 if you think it gets 6 barrels as part of the mod and each barrel needs the vent) and 100,000 for the facility to install it.
If you opted for the Vindicator, you can install the gas vent 3 as an accessory and still have all 6 of your mod slots for other mods. It will cost you just the 5500 for the gun and the 400 (or maybe 400 x 6 barrels depending on your GM) to have done so.
This makes the Ares cheaper in nuyen for the weapon but you still have to get the tools and you are down 2 mod slots over the vindicator. I'd opt for the vindicator at this point as mod slots are more valuable than money, or at least harder to come by.
And we haven't even touched the best damn LMG in the game yet. The Ingram White Knight with its weird gas vent 5 system and a price tag just 500 above the Ares MP-LMG, there really is no reason to look at any other LMG. Where else can you get an inherent 5(6 with stock) recoil in a weapon?