Doing compression testing is not enough, leakdown would then tell if there are any leaks to valves, seals, etc

Cost of AFR needed for boost does have negative effect of CAT live so a good test if possible is to swap both cats with a pipe and see what PCM reports via OBD-II

Being the engine is old with high miles it is possible CATs are getting plugged up. If car is less then 8 years old, at least here in the USA GM has to warranty CATs as to EPA smog warranty is 8 years

You could measure the exhaust temps in muffler and see if both sides are equal and also measure temps at exhaust primaries
using a cheap thermal infared tester

Still after resetting PCM engine ran better but degrades after PCM relearn miles then I'd tweak the tune being off a bit