Hmmm, interesting... The only #8 nib option I recall seeing was the US$230 gold (which did tempt me)
Does the #8 use a larger Bock housing, or is it just larger in the exposed area (I can understand it being limited to the Model 5 if the exposed part of the nib is too large for the smaller models). If it is just the exposed part that is larger, then asking for a "no nib/Bock threaded" might be feasible...
If you get a response (yes, I did see your follow-up) it might be worth posting here to let others know.
To answer your question, it's a larger housing, larger diameter, larger feed, larger everything. It won't go in a no.6 thread at all. Also as you may already know, Jowo and Bock threads are different. So it has to be made specifically for one particular housing size and type.