I'm theory if the car is running well and the cat material is of good enough quality, it shouldn't cause concern. Plenty of straight 6 cars have options for a single sports cats ie the below. I'll get the car tested and if whereby it won't pass, il swap out the system to OE spec. Few hours work on the drive once every 12 months is more than worth it for ART pipes.
http://cobrasport.com/index.php/high-flow-sports-cat-front-pipe-exhaust-2193.html
https://www.japspeed.co.uk/lexus-is200-2-0-98-05-exhaust-sports-cat-pipe