Makes me wonder why they didn’t make the ship strong enough that it was capable of sustaining 9.9.
They did; it’s called USS Voyager. Its maximum sustained speed was warp 9.975.
It’s not super obvious on-screen, but the Intrepid-class was considerably faster than even the Sovereign-class (Enterprise-E), let alone the older Galaxy-class (Enterprise-D).
The Excelsior used a different kind of warp drive. The Borg opened and traveled through “transwarp conduits.”