Battery standardization. But if given a do-over, I would probably go exclusively with 16mm plus spacers. 3 years ago, lacking this expert knowledge, I bought my 6Ps bored (from Bugsy). Fortunately I've had no problems.
At least my Z2 is stock!
My understanding is that there aren't many good 16650/17670 out there these days. The AWs are way too outdated (and obsolete), but the current Pana one (in the guise of Keepower protected, for example) must charge up to 4.35V to achieve the advertised 2500mAh.
By contrast a good quality but unprotected 3500mAh 18650 can fit into slightly narrower bores without issue, say, ~18.7mm, meaning the o-ring groove neck can be kept at slightly thicker (40-50C).
The other thing is that a well-bored C-body (6P especially) usually increases resale value. That doesn't mean one can make a fortune from that, however -- it depends on luck and hard work. The bore needs to be consistent, has upper side battery guard lip (some don't AFAIK), and optionally, reamed and honed for a smoother feel for battery sliding.
But NO, Z2/Z3/C2/C3 hosts are not ideal for bore mod. When a bore job goes wrong, so does the (precious) body concerned.
That said, HAIII reanodize is something highly suggested, as the stock HAII does't last for very long, before nicks, dings, chips develop in the long run.
But LiIon-wise, as long as the C-body's stock bore needs to be preserved, your best bet would be the Pana 16650 (2500mAh/4.35V).
