Wasn't there some kind of problem with the crystals; weren't they going through them at some accelerated rate and risked being stranded without slipstream? How was that resolved, or was it?
You'll find the answers in Unworthy.
Also, I thought the Luna-class wasn't much wider than the Intrepid. I don't see why Lunas would have a problem with slipstream. And IIRC the Sovereign-class is the same width as the Vesta and basically the same shape so it shouldn't have problems with slipstream.
The Intrepid class is about 133 meters wide; the Luna is 203.9 meters wide, 1.5 times the width of the Intrepid. The Vesta is 195 m wide and the Sovereign 250 m.
True, the Vesta's beam is very close to the Luna, but it's somewhat a matter of proportions, not just absolute width. A compact, narrow design works better.
Anyway, the bottom line is that the Luna and Sovereign are harder to upgrade to slipstream because we want them to be. The decision was made editorially that slipstream should be limited to the Vesta ships and the Voyager fleet for now, and the rest is just trying to rationalize that creative choice.
If I were to make up a handwave for it, I'd say that it's easier to keep the conduit stable the more the ship's mass is concentrated toward the central axis. If there are some bits sticking out substantially closer to the "walls" of the conduit, their mass could induce instabilities or something, and it would be harder to sustain the conduit.
Also, while these technical problems could theoretically be surmountable for a Sovvie or Luna given sufficient work and refinement, benamite crystals are as yet rare enough that they're being saved for more suitable classes. It might be theoretically possible to equip those classes for it, but it's not really practical to make the effort at present. Especially when it might be safer to wait a few years until the technology is better understood and its limitations can be more effectively overcome.
One would think that the Nova-class would be ideal for it.
Quite possibly.
Finally, has it been ascertained whether or not a slipstream corridor is round, or oval in nature?
Looks like a circular cross-section to me.
Hmm. Perhaps my memory is a bit rusty, but isn't that a transwarp corridor from Voyager's series finale?
Gatekeeper