Hmmm, I know "restricted lung hit" can be defined differently depending who it is, describing it, but since I know you have a Merlin, I'll compare it to that my Tube atty wide open is very close to the Merlin wide open, both of which to me are DTL, maybe very slightly restricted.
Mine also does not whistle at all, dual hole or single, possibly yours has some machining remnants some where causing partial blockage/whistling?
Yea great Frankenatty, huge thread over on FT with lots of ideas.
Airflow from least to most:
Monster V2, Monster V3, KF5, Tube.
V2/V3 are close, but V3 is noticeably more then V2, Kayfun 5 wide open is a fair bit more then a monster V3, but quite a bit less then the Tube atty.
What are you comparing it to? The Kayfun 5 is a good atomizer, not the best flavor but good. Not sure it even makes my top 10, maybe in my top 20. That may sound bad but I have somewhere North of 400 judge from.
So far I've gotten the best flavor from it with a 2.5mm SS430 .28g pulled maybe 1mm above the natural resting point of the coil. I've tried a fair number of builds in it, and so far I've not been overly impressed. Again, not saying it is bad, just not outstanding.