I'm wondering if part of the issue with no new IS OmniMechs using the tech that the Celestials used is because of the issues created with crit slots (or specifically, the lack of available crits after installing them) which apparently forced the WoBblies to deploy what were okay if not outstanding Omnis? You're basically forced to use small cockpits and
At least according to my copy of TechManual, it takes up six slots throughout the Mech... which means that, depending on the pod location, you've lost enough room to mount at least one weapon. While the Light Engine takes up fewer crit slots in the side torso than the IS-equivalent XL, you're still forced to figure out where to stick various things like DHS or jump jets (if the design needs them). Isn't that why the Seraph is stuck with an U/AC 10 and a snub-nose PPC in its primary configuration, which seems like relatively light equipment for an 85-tonner? Its HEAVIEST configuration, in my books, has the two Heavy PPC's and a light along with pulse lasers... and it only manages that because the lasers take so few critical slots. And this, along with the Deva, are probably the best of the Celestials for pod space.
Heck, isn't that why the Dire Wolf, the Clans' heaviest Omni, used Standard engines and armor?