My point is this. Here is the SNCB M6 cross-section:
View attachment 55690
Although designed for 760mm high platforms, the vestibule level is 1190mm, so the design would work perfectly well with a much higher platform level, which would also allow level access entry to the vestibule.
This will require rather more substantial steps to access the lower deck than found in TGV type designs.
This has implications for the interior layout and the speed of boarding.
As it is a TGV or TGV Duplex-derivative designed for GC gauge would be able to get away with only a single step from a UIC platform - which studies have shown is almost as fast as level boarding.
(EDIT: in addition, this profile will not permit a flat upper deck throughout the train, which will cause interesting problems for passengers moving around and trying to get in and out of seats. With the TGV Duplex style layout you can instruct passengers to board through every door and still have people move about above the possibly crowded interiors without clogging up stairs all over the place.
A flat upper deck is also interesting because it permits us to have a Class 185 style layout with lots of standing room and jump seat style seating down stairs for short hops, whilst having an intercity layout upstairs that is almost entirely independent of it.)
In this case we would end up with a full blown staircase to reach the lower deck, which is not tenable from a boarding speed perspective.
(Indeed with a proper 760mm platform, we could even get level boarding and double deck capacity on a GC gauge)