I'll freely admit that I'm not that interested in MHA and therefore don't hold much of an effective curve in my mind. But Deku literally outrunning Nagant's bullets after a fairly long distance after they had been fried was way above basically anything else in the series.
Him blitzing Shigaraki is him blitzing someone with inferior speed feats to himself, so it shows us nothing to indicate an actual upgrade because the distance was too small to matter.
He can crack mach, but he would have done that previously. Nothing to write home about, definitely not effecting the larger curve of MHA in my mind.
I don't see the argument that this is some kind of an upgrade.