That's not actually what the blog post Timo's referencing
said though. What THAT said was that the Symmetrical Warp Governor unit on the NX-01 was an attempt to workaround a problem which had plagued Earth warp ships since the Phoenix
itself (and quite possibly non-Earth ships for even longer, although the Vulcans presumably had their own workaround) - the fact that they couldn't produce a symmetrical warp bubble. The SWG manipulated the (dangerously) asymmetrical warp bubble to make it less asymmetrical, and allowing the ship to get to warp 5.
The same blog post says that that problem was solved some time not long before The Cage
, when they finally managed to generate natively
symmetrical warp bubbles for practical use, allowing them to get to the warp 6-8 range. Timo's idea involves them having to revert to a SWG at higher warps, well above the range they can apparently countenance an asymmetrical warp field in action.