I did 5 days and 100ish miles on the escalante river in UTAH in a solo tomcat. Had backpacker gear+ canyoneering gear including 60m rope and hardware. Paddled just fine loaded. Have run a solo tomcat as a tandem in a pinch. Was pretty decent. Had capacity for 2 people @175 each. Not a sportscar, but stable enough to get it done.
We rent them and so far have no problems about 5 years in. I do an annual bladder check and they they are fine. zippers run smooth, no holes, valves are fine. I thinks a solo has plenty of capacity for an extended trip. The tandems are far less maneuverable. when they go on trips the solos are snapped up every morning and the tandems get people cajoled into them.
I wouldn't be tempted to push into big class IV in a tomcat like some sportier duckies seem to do, so it is a compromise that way but I'm not sure you should push IV in a tandem lynx or similar.
Haven't paddled any other IK except a bandit which had far less capacity.
I've run cabarton, main, staircase and middle fork payette in a tomcat. Ran cabarton solo in a tandem tomcat. didn't care for it.