Yes, good question. Since
a) the exhaust manifold already has the 3 cylinder combined into one
b) the exhaust tube being very small
c) having two separate catalytic converter
That is enough for restriction and noise canceling. But it should make a drop in MPG. I think it will affect the ways the ECM computes the gas/air mix and will decrease the MPG
But you might gain some HP and torque
I think that as long as the new muffler holds well and air tight, you are OK. But is it all stainless. Your Insight is. And it is a nice feature.