In one of the old tech manuals I read, warp 8 was about 1024c. The old tech manuals also seem to say that power usage is about proportional to speed, so even if they could make the trip, they'd need to stop to recharge at about the same distance intervals.
Now, why they couldn't just go warp 9.9999 instead of warp 10 to avoid the nasty devolution effects...
That old tech manual actually explained why. The closer a ship approached Warp 10, the less efficient its engine became and the more power it would need to keep accelerating. Transwarp would bypass this in the same way a warp engine would bypass the need to travel at Warp 0.9999, IMO.