Since you removed the fuel cap and it made no difference it is not a vent issue, I would start looking at the return system any restriction there will cause a loss in power, you might want to remove the check valve in the top of the IP (the return line fitting) and inspect it, (careful not to get any trash in that port) it has a glass ball in it so you should be able to see light through it, sometimes trash plugs up that fitting. also you can carefully blow some air from that hose back to the tank to make sure it is not plugged.
You do have an odd issue, it will be interesting to see what you eventually find.
Others can comment on the effects of a plugged CAT other than loss of power, I would think there would be considerable smoke as the excessive back pressure should cause incomplete combustion as the cylinders would not be able to scavenge fresh air.