Moving here to not derail
Thinking about it a smidge more, I'm more fine with it thinking of the route like "The calc has them with four sea teas dodging at Mach 8.74, and we're just upscaling that since Sans/Flowey blitz late-game Frisk". That sort of ~15% jump is around the upper edge of what I'd accept, but isn't anything to get upset about really.The thing is already accepted, so you'd need to make a thread for it.
Plus Frisk being at least the speed that's above Tsunderplane during Sans' fight should be a given as they were filled with DT there obviously, and that many times showcased to increase speed, meaning that Frisk being somehow slower than how they were during Undying would be dumb.
Just know that 8-B won't be acceptable given that the gap is too large.