I'm guessing you don't have a tach. I mean...if you KNEW what the rpms actually are then it would be easier to figure out what is going on. The rev limiter will cause the engine to sort of "bark" and, yes, it can sound like it is popping. But it should NEVER hit the rev limiter if the outboard is in gear and has a properly sized, undamaged propeller.
Could your prop hub be starting to spin? That would cause you to hit the rev limiter and performance would be down.
If it's not that then it may be a VALVE timing issue. Your compression (160psi) is ok for a well worn engine but is far below the "nominal" of around 200 psi. A loose or jumped timing belt could account for the lowered compression and the performance fall off and popping...
The popping and performance fall off could also be because of an IGNITION timing issue. The ignition firing could be affected by a loose pulsar coil or a slipped magnet in the cam sprocket.
Another possibility is a weak or broken valve return spring or springs. This would keep one or more valves from fully closing and might get more noticeable at higher rpms.
Carbon build up around the valve stems can also cause valves to "hang" open. This is a problem on older engines that do a lot of trolling.
There are a few things to consider...especially if this is an older 20.
Good luck.