1. If you want Immeasurable Speed through the feat of going to another dimension prove these two things 1st. One this dimension is beyond linear time entirely. And two the character's in question get to sed dimension using only their own speed, no portals, no gateways, no special machines.
2. If you want plain old Immeasurable Speed no via crap, then you need one thing. Move through time using speed alone in a neutral environment, no special portals, gateways, or machines helping you.
3. If you want Immeasurable Speed through comparisons you need one thing. A Statement saying that moving through this special dimension of space requires a certain level of speed (Immeasurable Levels of it).
I agree they have Immeasurable Speed while inside the Boundary, but so far no one has actually shown they can do anything they can do in the Boundary outside of it.