The newer versions of it all have +5 listed.
Technically, I think all the listings with +5 are "wrong" because following the standard rules, the +5 should be added to the standard -1 of the heavy pistol for a +4 total (this isn't just a viper slivergun thing, almost all flechette entries replace AP with +5 instead of modifying it by +5).
While -1 AP isn't that big of a deal, it's really the principal of it that bugs me. It really should be +4 and -1, but it's not enough of an issue to redo the stats for.
You're really better off slapping AP flechettes in a gun with AP already. For example, the Super Warhawk would be putting out 8P(f)/-2AP with AP flechettes, and most battle rifles will be putting out 9P(f) with -1 or -2 AP. My personal favorite is the Elephant Rifle for a whopping 11P(f)/-1AP (12P(f) if you blow both barrels). Combined with it being impact armor (which is often lower for NPCs since they don't usually cap armor as much as runners), it makes things super deadly. And their is just something funny about military grade elephant gun rounds.