I was curious about them as well, so I grinded until I could get four Sirius AX Missile Racks (2m, 2l) yesterday, put them on my Krait Mk II and flew to Thargoid systems. My short conclusion is, that they are not meta changing. Better than normal AX Missiles, but worse than Guardian weapons.
In general, the upgrades are nice, more ammo and a faster rate of fire definitely feel better. But unfortunately the added jitter and the abysmal shot speed make this weapon pretty useless outside of short range battles in normal AX Scenarios. Hitting Scouts is possible, and they deal ok damage, but I often wasted a lot of missiles because of the jitter, which to be fair is small with only +0,5° but still noticeable on such small targets. Interceptors are easier to hit and hearts can be exerted rather easily, but destroying the hearts with the missiles is again rather difficult if you are not practically hugging the Interceptor. I only tested them against the Cyclops and Orthus variants and I had trouble fighting the Cyclops, but it is possible that I simply need more training with the missiles.
At the moment I think, Guardian or Salvation shard cannons do the same thing at the same range way easier and better. But in comparison to normal AX Missile Racks, they are better. They lose 3% damage but gain 38% dps and have more total damage thanks to the increased rate of fire, clip size and total ammo. Multiple Commanders could probably instant kill a Cyclops with them, like guardian weapons can. They might make a difference if one day we start an assault on the maelstrom systems.
I actually found one situation where they actually excelled and that is killing the new Orthus Interceptor, since it can kill all guardian modules on your ship with its overload wave rather quick. The Orthus is slow and does not move much, and I was able to solo kill it with my four Sirius AX Missile Racks and one class 3 grade 4 long range beam laser, without using premium ammo. To be fair, I dont know how well other weapons fair against it, but the missiles work well here at least. If the anti Guardian effect of the maelstrom spreads or the Orthus starts to appear in conflict zones, then it may be good to have one or two in storage. But otherwise they are just good to shake up your anti Xeno playstyle a bit. It is unfortunate that the engineering upgrade didn't triple or even quadruple the shot speed, that would probably help AX missiles the most.