So is this cooler a better performing cooler than the H115i?
We really need to see reviews to confirm this, and if so, by how much.
The mechanical weaknesses of hybrid coolers:
A)Pump speed. It pales to custom liquid pumps.
Plus, a larger radiator needs a stronger pump. If nothing is done about that, well... I really can't expect such a large model to best a 280mm - marginally better, perhaps.
B)Size of the copper fins on the opposite side of the cold plate. Bigger is better.
Do you add the cfm together to get 214cfm @ 4.18mmH2O since there are two fans pushing air for the H115i, and 218.4cfm for the Arctic?
I don't believe it works that way, unfortunately, I'm not the person who can answer this for you.
Though the specs you see only apply at 100%. Manufacturers don't post fan curves, so no one could tell you how a particular fan behaves below that; the curves aren't linear.
Does the lower static pressure of the Arctic (around 55% of the H115i ), mean lower performance?
Yes, when faced with 'obstacles'. The more there are, and the greater the intensity, the more air pressure is lost.
-the radiator itself
-the panel the fans are against, whether solid or open mesh
-cable clutter(if present)