All of my data/research shows the cherry bomb/straights to hurt power the most.

Thrush 16755, (Discount/auto) is quiet and cheap. Runs about 90db with 10.5/1 engines.
Nothing can touch the megaphone on the dyno or track tho.
My Son's next research is to look into the rules for Prod and mufflers and see if we can fill in the rear lower valence with a " muffler" consisting of the megaphone into the diffuser.