To me the NH-L9i looks exactly like the AR05, which is lower profile, and only two heat pipes. The AR06 is taller, and four heat pipes, which to me indicates it would have to cooler better.
I don't know what you mean by stack, but no, you can't stack CPU coolers as far as I'm aware.
I will try and get some temps from my HTPC today. I have temps but not overclocked temps, and honestly I'm not sure if they were the stock cooler, or the AR06, as I ran both on my G3258. I did check them when overclocked, but apparently I didn't write them down, I remember it being perfectly acceptable though. Since I wasn't exactly sure when those temps were taken, I didn't want to post them in your other thread. Unfortunately that machine is now an I5-4590 which will create more heat, but we'll see what I get anyways.
As mentioned in your other thread though, I don't think your RVZ01 case has as much air flow as my ML03.