Onslaught is flux starved. Heavy Needler in the center heavy mount is not a bad idea if going for a dedicated anti-shield loadout (against something like Radiant). Three heavy needlers in the mediums is not quite enough anti-shield against a Radiant. Four is better.
Mark IX is not efficient enough (and inaccurate without Gunnery Implants), and Storm Needler does not have enough range. Sometimes, Heavy Needler is the best kinetic in a heavy ballistics mount.
If Conquest did not have Heavy Ballistics Integration, I would use Heavy Needler instead of Mark IX on Conquest.
Sometimes, on Dominator or Legion, I use Mjolnir and Heavy Needler (in the heavy mount) instead of HAG and Mark IX.
Edit: will heavy Needler spam still be viable in 0.95a? Probably not, but I still find it much more preferable to installing any large kinetic weapon on the side mounts.
Remember that Alex may make Mark IX more efficient, and Onslaught will get Heavy Ballistics Integration. Also, the side mounts will have their arcs changed (shortened if I remember).