As King Daniel
will attest, the topic of just what the hell transwarp drive is pops up every so often in the Tech forum. It's remarkable how little sense the depiction in this episode actually holds. As others have noted, the warp scale was recalibrated at the start of TNG so Warp 10 was an unreachable speed. (Apparently the production team was sick of the notion of the ship accelerating out of control as on TOS.)
What's interesting about "Threshold"'s depiction of transwarp is that it does a pretty good job explaining what the technological limitations might be to achieving such high velocity, but alas, lizards and all, it just doesn't hold water.