It's not TH+8 in gear that is the cap but TH8 including gear and traits.
It's understood as you said, that wearing higher TH increases you likelihood to proc a TH upgrade. What you failed to mention is the data leaning heavily toward working on procs with TH9+ not having any further impact on upgrade rate than just staying TH8.

As such, if you just tag an enemy with TH8, then swap to TH3 for the rest of the fight, you are unlikely to get any/many procs at all. If you full time TH8, you should get some upgrade/s depending on the duration of the fight.

At least this is what I have taken from the thread on AH.