Straight DPS comparisons are missing the broader picture here, so I'll have to go more in-depth on the topic.
To help with comparison and balancing, I've made a spreadsheet with turret data, you can find it within the mod's folder (Factorio Turret Sheet.ods). I'l be using DPS calculation from there to help explain why I've done balancing in this particular way.
Assuming yellow ammo and no damage techs for now, Frontline HMG has raw DPS of 60 vs Gun Turret 50, about 66% more range and 275% more health + armor. Doing twice the damage per bullet also means 50% less resource drain from ammunition expenditure, major upside in the early game. Downsides are more than twice the cost (115.5 raw units VS gun turret's 50 raw units), more complex manufacturing and vulnerability to being flanked due limited firing arcs, so it cannot be placed haphazardly like gun turrets can. At the point it's being unlocked, this is mostly a straight upgrade to the basic Gun Turret.
As damage tech increases, Gun Turret scales DPS much faster thanks to the multiplicative damage from the extra +TURRET_DAMAGE effect. Let's compare the raw DPS across the tech phases, using yellow ammo and base game as a reference point and researching all +damage and +fire_rate techs available up to that point:
No techs: HMG: 60 > GT: 50
Red: HMG: 72.6 > GT: 66.6
Green: HMG: 93.6 = GT: 93.6 - Break even point, and likely it's here HMG would be unlocked once you got gray science
Gray: HMG: 172.8 < GT: 230.4
Blue: HMG: 226.8 < GT: 340.2
Yellow: HMG: 330.0 < GT: 605.0
If you're basing your opinion on JUST DPS number alone, I can see why you'd discard HMG when infinite techs are in play. But let's go further. How about ammo economy? How efficient these are? Metric here would be DPH - Damage Per Hit. Same assumptions as above.
No techs: HMG: 10 > GT: 5
Red: HMG: 11.0 > GT: 6.1
Green: HMG: 12.0 > GT: 7.2
Gray: HMG: 16.0 > GT: 12.8
Blue: HMG: 18.0 > GT: 16.2
Yellow: HMG: 22.0 < GT: 24.2 - Gun Turret takes lead
Here, HMG is more economical all the way until all pre-infinite techs are done. Well, you might ask, why keep the now inefficient and outgunned HMG at all then? What's the point?
My answer is RANGE. Let's have a following scenario - you're post game, at Physical Damage Research 12, which gives (in base game, Space Age is much lower) +360% bullet damage and +540% gun turret damage. With uranium ammo, that's whoppig 17668 DPS per Gun Turret! What happens when a group of, say, 20 Behemoth Spitters attack a line of 15 Gun Turrets?
Gun Turrets have 18 range. Behemoth Spitters have 16 range. When Spitters attack your defenses, Gun Turrets have a small lag due to having to deploy into ready position first, it takes about half a second. This means both parties will basically exchange damage with each other. Spitters die near-instantly, but you're likely taken attricion damage from the exchange. I my testing of this exact scenarion, I lost 4 turrets.
Now, 20 Behemoth Spitters VS a line of 10 Frontline HMGs. From our tech, turrets have a DPS of 3312 with uranium ammo. So much lower that Gun Turrets! However, HMGs have 30 range. Spitters have 16, meaning they have to cover 14 tile distance first before they can shoot. Spitters have movement speed of 32.4 km/s in the tooltip, which translates to 9 m/s. I assume one tile has length of 1 meter, which seems about right from in-game measurement with a stopwatch. It takes 14/9 ~= 1.5 seconds for them to reach the firing position. What happens?
Well, Turrets have DPS of 3312. Spitters have 1500 HP without any armor. Turrets have pre-targeting, so they are aiming and ready to fire the moment Spitters get into range. Time to kill a Spitter is 1500 \ 3312 ~= 0.45 seconds. Meaning each HMG turret can kill 3 Spitters before they even reach the range Gun Turrets could fire at! Again, tested this with editor mode - all Spitters died before they even got to their firing range.
Now obviously, you could say "What happens if you throw 5 times that much enemies? Surely some of them would survive and deal more damage total that if it were Gun Turrets instead?". And I'd say... Yes. You would be right. But that is exactly the point I'm trying to make here - that these turrets DO NOT OBSOLETE EACH OTHER. Both have their place, their niche to excell at. When I look at balancing turrets for this mod, I don't balance VERSUS each, I balance IN AGGREGATE WITH each. And it only got better with Factorio 2.0 adding targeting filters - use both at once! HMG with target filtering to kill Spitters before they can damage your squishy Gun Turrets, while those with target filter for Behemoth Biters to shred them with vastly superior DPS.
Same thing happens with other turrets in this mod - they are made to complement, not compete, with one another. And, trust me, you don't really need any damage techs for these things, nor quality ammo for that matter. With, frankly absurd, +100% damage for cannon shell each tech, cannon turrets alone (with explosive shells) absolutely wreck face, one-shotting behemoth groups with each shot. Back in my 1.1 save with Krastorio 2, while late game and dabbling with matter techs, I threw Rampant in just for fun, to check how my defenses would fare. The only adjustemt I had to make after like 2 hours of playtime after adding Rampant was to triple rocket and cannon shell production to keep up with demand, and that was it. With defensive line being, in order of engagement depth (first to last): Cruise Missile (B), Heavy Cannon (B), Dual Cannon (B), HMG (F), Heavy Scattergun - biters could barely reach to HMG range, if that. I was on Optimisation tech cards for damage techs at the time.
Modular Turret specific damage techs are really not needed, unless you throw some insane modded nonsense in, like godzilla-tier boss monsters that are near immune to everything. But hey, you got other mods to cover that, I'm not going into that territory.
