If Onslaught cannot sustain Storm Needlers for the full DPS, then what is the point of using them instead of Mark IXs (or smaller kinetics) that it can sustain for less OP and better range? Low-tech ships have more mounts than their dissipation can support.
Because when shooting into shields with kinetic weapons the ability to sustain firing does not matter as compared to the ability to put DPS out. We showed this in one of the threads discussing optimal vent/capacitor allocation and found that the actual optimal fit for situations where you're shooting kinetic damage into shields is that you max capacitors before vents.
Plus the storm needler is more efficient so when it is capped it can shoot less and end up doing more DPS.
The storm needler does 26.7 DPS/OP at .86 flux/damage. The Mark IX is 19.3 DPS/OP at 1.15 flux/dmg. If these numbers alone do not give you a reason to fit a storm needler then i am not sure what does.
The only reason (besides range) i would not consider putting a storm needler on an Onslaught is because i want the large ballistics for HE in the form of HAG's. The Onslaught cannot sustain the HAG's either but the inability to sustain the flux use of two HAG does not matter.
I suppose that if the Storm needler does have an issue its range. Its legit hard to fix a storm needler at 700 range because you're giving up a 320 range advantage to Mark IX and 160 to a Heavy Needler. Another problem might just be its raw OP cost. Its legit hard to fit a storm needler at 28 OP.
But even then... the main issue is that most ships that have large ballistic also have medium ballistic. And if you have both then you want kinetic in your mediums and HE in the larges simply because large weapons have better dmg/hit in general and that makes them better.
So what kind of ship would get a lot out of a storm needler?
Well a ship with a large ballistic(or universal) mount that had medium energy mounts instead(and if it had small ballistic needed them/wanted them for PD). If the Aurora, instead of its 4 small and 1 medium hybrid in the front had a large universal or ballistic the Storm Needler at 700 range Kinetic would be a
strong contender for that mount. Since you're going to finish things with HB in the two medium energy behind it and since a 900 range HE doesn't do you any good without a way to puncture shields until the 600 range HB in the medium energy...
edit: This is using the old stats of the Mark IX (which uhhh. probably didn't need a buff i think the mark IX is a perfectly capable weapon). At 1.0 dmg/flux vs 1.15 this doesn't change the calculus all that much.