Kona - I'd say it would be worth asking in a Kwik Fit or Fast fit or some of these guys, they'll be able to tell you very quickly if its doable and how much it would be.
Wrt to just getting a bf or mongoose downpipe, got a mongoose dp myself, and noticed the difference straight away. Revved easier and got more torque (haven't put it on rolling road before or after but it felt like I had more anyway), so all in all I was happy with it.
The stock downpipes are poorly designed from a flow point of view. The angle the pipes from each side of the V meet isn't good, whereas the BF or Mongoose ones have the angles much better.
Here's the stock vs the Mongoose for direct comparison:
Wrt the RPW headers, they'd prob be better again, but you'd pay a fair bit for them I reckon.