Hello George,
You lose your warranty going below 11.25mm. The lowest that I go is 11.3mm.
This gives the client a smidge of insurance.
Changing the taper on a Z shaft will dramatically alter it's play characteristics.
18" is a bit extreme in anyone's book, especially @ 11.3mm.
But, it's not my shaft so I don't care. I'm just trying to answer your questions.
The shaft will stand what you want to do, you just won't like how it plays.
Do I have any experience with this shaft?
Yeah, I've done a few.
KJ