If I had it to do all over again, I'd go with the ATR 3" single shot. The limiting factor in exhaust flow is your DP. That is where the exhaust gas is at its’ greatest tem and therefore its’ greatest volume. Going duel 2.75” or 3” is not going to make a difference over duel 2.50” or a 3” single shot. The 3” will out flow duels because most of the loses is from the surface frictional loses. Plus the 3” of course weights and cost less. As for sound, I’ve heard that the 3” is VERY loud.
How that helps,
Jason