This is an alterate universe, so size and speeds from the prime universe don't apply here.
Wait a sec here. It is an alternate timeline
, but not an alternate universe. Nero's incursion didn't alter the laws of physics. Whatever the "formula" for warp speeds was before, it is now as well.
That's not to say there was ever any standard for warp speeds before, but to suggest that the timeline divergence would change that is bunk.