Positive lead diagnostics on YouTube used the HF vacuum tester with a bored out old o2 sensor. It lead me to go by the $15 tester. Seems like a cheap option for something I don't use much.
Converter is the likely cause of the exhaust flow ......drop the converter and visual inspect.
Honeycomb substrate melted or damaged......easy to see with the converter inspected and more conclusive than the gauge tests. Gauge test will give good results if the flow is low.