Heavy needler on the other hand, I don't see any reason not to take them over Heavy Autocannon. The do much more damage. The +5 OP cost is already refunded in flux efficiency. ( -50 flux is equivalent to 5 Flux Vent). And it's better burst damage and more accurate. And does more DPS if medium mounts are limited. The only disadvanage is that it would do less damage to armour/hull, but its way undercosted.
I do not agree with it being underpriced. With its OP cost, I can only mount two needlers instead of three HACs for the OP spent. In effect, I trade DPS and steady suppression for better accuracy, efficiency, and getting a flux spike at a bad moment. I think Heavy Needler is in a good spot now. The only anomaly with Heavy Needler is it has 800 range while other needlers have 700 range. (If Heavy Needler gets 700 range with no other changes, I will drop it for Heavy Autocannon in a heartbeat, unless I really need to match Heavy Mortar's 700 range.)
Of course you would think think Heavy Needler is in a good spot now. It is a much better choice and is more OP efficient than Heavy Autocannon. You did not point to a build where you would prefer to use Heavy Needler over Heavy Autocannon.
Lets us see what the cost of a Heavy Needler should be using the Heavy Autocannon as a baseline. The Heavy Autocannon is 10 OP for a 214 DPS and 214 Flux/second.
The Heavy Needler has 250 DPS at 200 Flux/second.
In terms of proportionate DPS, the Heavy Needler is 250/214 so is +16% better DPS, and so the proportionate OP cost of the Heavy Needler should be 11.6 OP.
However, a Heavy Autocannon, taken to that proportionate DPS would use 250 Flux/second. As the Needler uses 200 Flux a second it has essentially has -50 Flux/second than it would do proportionately.
-50 Flux/second is equivalent to 5 inbuilt Flux Vents, which costs 5 OP.
11.6 + 5 = 16.6 OP
So the Needler is worth 16.6 OP. This is not even taking into account that the Heavy Needler is more accurate, and sharp bursts of fire is generally more desirable than steady dps. Or that medium weapon slots are a limited resource make the heavy Needler worth more than that. We can say that as the Heavy Needler is less effective against Armour, they equalise. They don't as the far greater accuracy and burst fire is better than a slightly lesser ability to affect armour for KE weapons but we will do so as it is hard to quantify.
Let us take another avenue of approach. You have 3 medium weapon slots. 3 Heavy Needlers is 750 dps for 600 flux/s. 3 Heavy Autocannons is 642 dps for 642/s. If somehow taken proportionately its clear that the 3 Heavy Needlers are the more OP efficient cost. For the 3 Heavy Autocannons to be worth the same dps as the 3 heavy Needlers, they will cost 35 OP, but then uptake 750 flux/s, costing an extra 15 OP to reduce the flux cost down to 600 flux/s. The 3 heavy needlers are much better.
Or let us compare your approach. 2 Heavy Needlers with 3 Heavy Autocannons. 500 dps for 400 flux against 642 dps and 642 flux. Looking this the 2 heavy Needlers already look superiour to the 3 Heavy Autocannons, but let us analyze it anyways. Firstly the 2 Needlers have a spare medium ballistic weapon slot. Taking the 3 Heavy Autocannons down to 400 flux cost will cost 24.2 Flux Vents. That's enough to fill that empty spare medium weapon slot with an additional Heavy Needler and 9.2 Flux vents. For 750 dps and 508 flux. Those Heavy needlers are looking so much more cost and mount effective than Autocannon.
Again I'm not even taking into account that the Heavy Needler is more accurate and has better burst fire. So it's worth far more than that.
If you wanted low OP kinetic, in most circumstances it is better to go for less of dual light autocannon than multiples of light autocannon. Or simply replace the light autocannon. They are 1 OP difference and it is most likely a better use than whatever that OP was going to be used for. Can you give an example circumstance? Even the colossus MK II/III will be better off going for dual type instead of single type.
Either if I have a single mount and hurt for OP (like Shrike), the mounts do not cover each other, or I cannot support all mounts with 140+ flux weapons. Yes, it is niche, but not useless. Light Autocannon could be boosted somehow.
Or, just take that single dual light autocannon over the light autocannon? It's 1 OP more and for that you are gaining +42 KE dps. It's a mistake to take a light autocannon rather than the dual light autocannon under the impression that you cannon support the flux. Post your shrike build and then I'll make it better by removing 1 OP somewhere else. The Shrike is a fast destroyer with massive flux stats and a ship system suited for building and dumping soft flux.