I think Federation calls anything beyond their normal warp technology 'transwarp'. Exelsior's engine's were considered transwarp engines at the time (and were not a failure*) but once the technology became widespread and commonplace the 'transwarp' became to mean warp techologies even beyond that, such as the Borg warp drive. (* The adoption of this new type of warp engine was the reason why the warp scale was changed.)
As good an explanation as any.
I think they need to do something new with the existing post-TNG/DS9/VOY warp speeds. Warp 9 is fast. Warp 9.5 is bloody fast. Warp 9.9 is hellafast. But now we're in Warp 9.975 and such territory. And recently in Trek literature we're into quantum slipstream drive. All of the Warp 9-plus numbers are getting blurry. I think even the slowest Trek ship can go, like, Warp 6. Time to redo the scale again?