Your modifications are relevant to my interests.
How go the new frets? I've played some 12-string acoustic and electric guitars before, but I have just recently this past autumn acquired my first 8-string bass. Even though I'm not using a low B, I know what you mean about the relative difference in diameter between the root and octave strings. Much more noticeable on a bass than a guitar.
Doing the kind of work you did to the frets seems like a rather radical fix. I would love to try it out, but don't know that I would spend that kind of money on it without trying it first.
My first thought was to swap the root and octave strings -- it seems to me it would be easier on the fretting fingers if the fat string was above the thin one. That would required altering the nut and possibly the bridge (or saddles), and, in my case, a little work on the tailpiece/string anchor. But that seems like a lot lees work than radically modified frets. Have you given any thought to that idea?
I think pretty much every 8-string (or 12-string) manufacturer has the octave string above the root string except one -- and I can't remember which one. Maybe Rickenbacker? Has anybody played both types, and have any comments on the difference?