Allowing a pen +2 non-primitive bow would be nice, having some modern kind of accelerator or whatever. Explosive arrows would have to be buffed then though (giving tearing and blast maybe ?).
Unless you houserule that thrown mono knives lose their Pen, I don't see why you'd need an accelerator to replicate the +2 Pen on "mono-tipped arrows" - although it sounds like an option, similar to the "pneumatic" thing that was used as a way of adding Mono to hammers.
As for explosive arrows, Tearing sounds good - it'd be a nice tie-in to bolt rounds and actually bring a bit of system into what looks rather arbitrary now. Maybe add a small raw damage boost, too?
Edited by Lynata