Here's the clearance I have with 25mm GP4000SII in the front. There's enough room in the back.
If the new-gen Veloflex uses the same sizing standard as the old-gen, 25mm would get me much smaller tire than I would want. (Considerably smaller than this one pictured).
Ideally, 28mm Corsa Evo would be the same as 25mm GP4000SII, or a tiny bit taller, since I'd be comfortable with even a bit tighter clearance than pictured.
In case they changed their sizing standard from the time they produced the Master 25, I could end up with too big of a tire that I can't fit in the front, if opting for the 28mm. Maybe someone had experience with both old and new generation of Veloflex. I believe the old Master 25 measures 66mm of width, unmounted and laid flat, from end to end.