450 SL Exhaust Question

having a 450SL exhaust manifold replaced, it is the type with
the "built-in" catalytics. The mechanic wants to use 380 exhaust
manifolds and "increase" the size of the 3rd catalytic.
Is this doable? downside? He says this will reduce the noise level
and heat buildup in the engine, but otherwise have no effect.

Thanks

akas