Don't they have tungston flechete in War! that has better armor defeating traits? Wish I had my books 
Yeah, and they use Impact armor, not Ballistic -- but they're also availability 18, and cost 16 nuyen a round.
Personally, I've always liked the idea that the Viper isn't just another pistol (that happens to shoot flechette). This is nothing canon, and no book says so, but it's
so much better than other heavy pistols in a few key ways, that the explanation that makes sense (to me) is that it just doesn't work like any other gun. It's got the rate of fire that it does, it's got the magazine capacity that it does, and it's got the built-in lack of sound that it does, by being something besides a traditional firearm. The "magazine" is some sort of solid polymer brick, and the gun fires by scraping slivers off that brick and hurtling them down the barrel a whole lot of slivers at a time.
It's not so much a generic heavy pistol that happens to only shoot flechettes for some reason, as something completely "out there" compared to traditional gun designs (a specialized needle-gun in a very cyberpunk or even sci-fi sense)...in my head. I don't know how else to explain why it has double or triple the average heavy pistol ammo, a better firing mode, a built-in silencer (that you can't see in any picture of it), and all for the same size, otherwise.
So that's part of why I, personally, dislike tweaking it to get even more out of it, or to make it fire regular or other upgraded ammo. In my brain, all it's got in common with a Predator is the company who makes it.