I went from a 268XE retarded 5* to a 286 Magnum installed straight up and the exhaust is substancially louder! Is it becuase the new cam has more duration or what?

Ya, try an Ultradyne/Lunati, not only it is louder, but it resonates the primaries something fierce!

Think it has to do with duration + exhaust gas velocity, and louder is better I think! Let's see if a pro can confrim that.

Both the longer duration and the fact that the cam is advanced (relative to the old one) contribute to the exhaust valve opening sooner. This means that the residual cylinder pressure is higher when the exh valve opens. Thus, more "pop".

Louder exhaust sound (let's say actually defined sound) is what you should expect. You added 6 degrees exhaust and moved the cam forward 9 degrees (TDC + 4 deg built in) plus the 3 degrees additional in the cam timing. The new cam is opening the exhaust valve about 12 degrees sooner, maybe more if your timing chain was worn. You have a longer intake duration also - so more air/fuel to burn = more air/fuel to exhaust.