MEKer,
if you're looking for a fast neck, don't immediately jump to the narrowest 5 string neck available. All of my 5 & 6 string basses have 3/4" spacing (4-string spacing) and are fast because of the NECK PROFILE. I use wide necks that are thin. I've found narrow necks to be a disadvantage in several situations and not necessarily faster. Try a wide 5er with a flattened C neck shape and I think you'll be surprised. Some basses with this configuration are the Fbass, Modulus wide 5, Sadowsky, Status, and many others. Also, don't forget that the ergonomics of the bass (balance, comfort, etc) have a lot to do with one's ability to shred on a particular instrument.