Well it could be the in game its warp 10 and not warp 13 which i think is canon in st agt but i don't know if that was the maximum warp speed of the refited galaxy?
Commonly accepted speed of the Galaxy class is around warp 9.96 (unsustainable) with a cruising speed of 9.2 after her various improvements. The Galaxy-X we saw in AGT was at least Warp 13, as you've stated.
But certainly, as of Nemesis, the Sovereign class held a pretty commanding top speed of 9.97 sustained, if production notes are to be held with any canon respect. (Notable that this figure is the one used in game as the top speed of a ship at Vice Admiral 1)