Back in the 1950's. some hotrodders installed exhaust cutouts in their head pipe. A short piece of exhaust pipe was welded in, then capped by a 2-bolt exhaust flange. Some used a gas cap to open and close the port.
Others controlled the cutout by a choke cable.
A cable might be a nice way to control your exhaustive innovation.
It changes. For me it is about 1000mi. But then my setup sees much harder stops. I also have to change out my disc every other pad change. I run the metal pads on a 4-piston. They eat the disc. Goes down to .010" under in the middle of the braking surface of the original .070 or whatever disc thickness.Replaced my front disc brake pads. Have used them about 4 months. Not much longer life than the Salmon Kool Stops I used on the calipers, but it certainly stops much better with the disc.
What is the average life of a bike disc brake pad? This is the first application I've ever used them ---- motorized or not......