Without any evidence that the scale has been recalibrated, we must assume it has not been.
Why must we? Given two possible hypotheses;
1) the scale has been recalibrated
2) the underlying nature of physics and mathematics have altered to allow "greater than infinite" speeds - whatever that may mean
the logical choice is pretty much a no brainer - good old Occam's Razor makes the call for us, especially as every portrayal of transwarp (other than "threshold"-which is clearly shown to be a different beast to other transwarp iterations) has shown finite travel times and therefore sub warp-ten velocities.
However fast Borg Transwarp is (or slipstream, etc) it could in theory be measured using the conventional scale in terms of WF 9.9999....whatever, that much is clear, it is NOT "faster than infinite".
Therefore unless the universe has simply changed in the years leading up to AGT then the likeliest scenario (and strictly speaking the logical hypothesis unless you can offer a third option) is that the meaning of "warp 13" has changed instead
In either case, even if;
1) nacelles operated as the ship's power source - which they don't,
2) the number of nacelles had a linear relationship with power output - which we have no way of knowing or frankly even guessing (if so, why not build ten nacelled ships?)
and
3) subspace and it's physics have suddenly started to allow greater than infinite speeds (it hurts each time I type that - and it hasn't)
It still does not explain how giving the ship a 50% increase in power would allow it to achieve those logically impossible speeds.