Try wrapping the perforated baffle core with tin foil, this should help the exhaust gases find their way out as briskly as possible...

I have the same zingy sound aswell.
I had a thought though, it makes that sound at around 5k because of the vibrations coming from the engine and shaking the headers because it is a thinner material than mild steel, sooooo wouldnt it be an idea to get 3 pieces of stainless steel braces welded on to hold each header, i.e 1 piece going from 1 to 2, 2nd piece going from 2 to 3, 3rd piece going from 3 to 4.

If i a right then this should get rid of the zingy vibration sound all together because it will prevent them from rattling, instead they will be all in a solid postion.

Correct me if i am wrong.

If this would work, were would do stainless steel welding and do a good job of it?

Adam