3 inches is too big for that motor unless you are running a single exhaust.
You want to keep the exhaust gas velocity high.
Folks who say 'no such thing as backpressure' are off base IMHO. Why not just run without exhaust manifolds or headers at all? Now that would be zero back pressure for sure. Try running a motor like that and see how well it runs.
Big pipes are of the 'more is better' train of thought.
In my bike shop guys would come in all day, wanting fatter and shorter pipes. And then wonder why their bike lost all their punch.
Rant off, sorry 'bout that, but my vote is for a pair of smaller diameter units.
