I'm saying you can overrestrict, but you can't underrestrict. And if you buy a normal-sized aftermarket cat, you aren't in danger of overrestricting. These things are not that picky.

FWIW, I spent a decade fucking around with the exhausts of seven different POS lemons racecars. I've fucked around with every possible cat-delete configuration on a bunch of japanese and american cars. Modern emissions systems are really really tolerant and have enough map range to accommodate almost anything an amateur mechanic can throw at it. These are not high-strung race cars operating at the edge of their envelope. They're grocery-getters built to keep running fine under all kinds of environmental and maintenance conditions. Chill.