As you were saying that pick is great in 5th I hardly suspected the CAT to go bad. Usually when the catalytic converter is clogged, your car rocks at a certain speed like it cannot go beyond that speed. It feels like limited to a certain speed.
However, as you are facing the issue in lower gears, it can be fuel filter or pump failing. Have the pressure checked for peace of mind as well. Also if your current filter is older than 70000KM have it changed.
Car slowing down after release of accelerator is okay, its engine braking, however as you suggest that you have to put mettle to the pedal to accelerate it again, points out to poor fuel delivery by the pump. Again, pump issues can be filter clogging as well.
Also have your tappets checked as well as have the throttle body cleaned. Just a simple maintenance task that needs to be done after 40K km if you have not done it.
As you said the MAF is clean, but it looks clean as its after the air filter. usually tinny specs of dust that are not visible can reduce its performance. If you have had never cleaned them, try cleaning them, you will feel a very light car after that. (Do not touch the wires on the MAF sensor as they are very sensitive and can break easily).(Also only use MAF cleaner that is specific to cleaning MAF, do not spray anything else or you will destroy it).
