Not speculating, just curious as a larger value opening should help more. If that link from bbq guru hold correct you can have two equal 1 1/2's for aesthetics purposes as well as efficiency.
How did you tweak the slam latches to be that sealed. Looks as if the exhaust ends are different now than when you bought yours. Seems to be one built in the middle instead of a slide at the top.
Team Smokin' Hot Racks, Assassin 28, 2 L BGE's/Weber 26.75/Guru DX2 & Wifi/Thermapen Orange!