Here is my air flow experience with the M range.
2018 very restricted.
2019 less restricted, perfect for me.
2020 very free flowing, not my jam but great on a water peice.
2021 somewhere between the 19 and the 20.
Of my two daily drivers I tend to use the 21(captive) more as a session vape and the 19(standard) for those quicker extraction needs, although both can do either with appropriate techniques.
It must be said though.... the cap makes a big difference to air flow with the standard cap being more restricted and the captive increasing flow.
Can't speak for the gorgeous phantom coating, but whenever I want my DVs sparkling clean I give them a soak in Powdered Brewers Wash followed by a thorough rinse and they come up beautiful.
You could test the PBW on the phantom's condenser before you risk it on the main bits.
(don't soak the cap)
Never tried a low temp cap but the perf cap seems to extend the session and give a bit more airflow. It also looks awesome.
I think the Half Bowl Converter is just the right height so that it supports the ccd as it "clicks" into place, therefore eliminating the
ing around trying to get it seated properly. The HBC also adds some mass to the tip but I have no idea/experience what effect it has on performance.
Happy vaping.