IT took me a second to relaize you were asking a different question than what I read (Its Friday and I am tired)...
I could see either +4 or +5. I was never under the impression that Byeshk was more rare/useful than Adamantine as far as combat applications but then again it is "talked up" a lot in Eberron so it might warrant the +5 just for rarities sake.
Byeshk is priced closed to cold iron/silver than adamantite. I'd stick with +2/+3.