Plugged cat will cause loss of power, but the downshift with some power return does not sound like a plugged cat - the increased gas flow at higher rpms will just jam it up worse. If the depression chamber is bad, the secondary just won't open, and you lose no fuel that way. You may get bad mpg due to trying to accel without enough air and fuel, leading to very long accel times in a lower gear, but not as bad as the carb running dead rich.

Not responsible for advice not taken...