I've rolled 16ga with my cheap 3 in 1 roller, brake, shear.
It's rated @ 18 ga mild steel.
You just need to be patient and work slowly.
I assume you need the roller just for the resonator, and build the header itself using mandrel bends.
16ga mild steel or 18ga stainless would be my choice of materials, resonator could be 18ga MS depending how short the pipes are before the collector, i.e. "how hot does the resonator get?".