To be honest in all my years playing alembics i just got used to whatever taper the neck has. My series 5 stringers all "feel" like they have the same neck parameters, taper, width and back to front depth.
That may not be true when measured but it doesn't feel any different when playing. My standard Bocate Europa 5 feels bigger in width depth and taper. My Black Europa 5 has really noticeably narrow neck width and taper, and feels deeper front to back and is the easiest to play finger style but hardest to slap. My theory is that the original owner was a metallica fan and probably planned to use it with a pick.
I think if i was going to have custom 5 string I would choose a narrow taper like the black europa but not as narrow as Jimmy J's basses. :-)