Interesting post, this is one of my unanswered questions as well.
I played with a 12.25 or so Schon shaft for years, then switched to a Z2 (11.75), and now am working with a 314-2 (12.75). I like the more solid feeling of the 314-2, now the Z2 feels like a toothpick. But I did like the lower deflection and english adjustments with the Z2. A dilemma for me for sure, I can play well with both, but switching back and forth is definitely bad...
In general, don't most top players tend to stick to the "normal" range, 12.5 - 13mm or so? I didn't think there were a lot of players that were using Z shafts or similar small shafts, but maybe I'm wrong. I guess whatever works is fine, shouldn't matter. I often feel I would like something in between, around 12.2 - 12.5mm, but Mezz is the only standard option, maybe I'll try one of those. I'd love to take my 314-2 down to 12.5, but I think it can void the warranty and I'm not sure what the hit would be like, an expensive experiment if it doesn't work out...
Scott