I run 1.625" primaries pipes into a 3" merge collector and a twin 2.25" mandrel bend stainless system. When I removed the cats (200 cell) the car picked up a little over 20KW at the wheels. The primary pipes didn't glow red under a full power pass afterwards so there must have been a significant restriction. But they were only cheapo metal cats.
If its something you really want to do / try, get a set of straight through sections made up with flange plates to replace the cats. That way you can put m back without exhaust mods.