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.
Yeah, but there's in-universe reasoning and then there's making-a-movie justifying. The whole point of throwing away the good universe in favor of a new one was to let the new kids play in their own sandbox. That means they can fill it with mud if they want and still call it sand -- which is how you get Delta Vega, a world on the rim of the galaxy, to migrate to within spittin' distance of Vulcan, I suppose.
Or build starships on the ground because the nacelles have to be calibrated in a gravity well (believe it or not, I remember reading the screenwriters spouting that one in print.) I think that's a whole lot more stupid than doing a black hole movie and employing 'simply irresistible' as the theme song.