Don't know about the 25", but 28 5/8" - if I see this correctly - is as if you took a 25.5" scale fretboard and added two more (lower) frets behind the nut.
However, I was wondering why shimming the neck didn't help. I have no experience with Kahlers, but the angled neck pocket for a non-recessed Floyd is indeed barely visible. Also, I just recently have used very thin pieces of veneer as shims to angle a neck quite a bit, so just out of curiosity: What did you do and what is a "stupidly thick shim" in your definition?