If you really want, you can put dual coils in a rm2, I have duals in 3 of my rm2s[emoji2]
It's the RM3 for me. Dunno why, but I don't find the small deck a problem to build on and it kicks (and I mean kicks) flavour.
The RM4 is nice but too airy for me (and I'm a mouth/lung draw kinda guy, not a direct lung hitter).
As for another BF atty, I like the cyclone (which you can get BF enabled from the factory, so I'm not sure if it fits your criteria for BF modded). I find it a very nice atty with AFC.
I like the RM4. I use a regular driptip rather than the cannon it comes with and i find you can regulate the airflow to either mouth to lung, or straight lung and cloud chase on it. I also reckon that you can get a 'smaller chamber feel' to the RM4 if you tuck some wick under the coil(s), so it's quite versatile with a little effort. Of course, running dual coils at 0.3Ω will rasp through any batteries and so i don't run that if i'm going to be stranded somewhere dismal (like work say) for a period of time.
For single coils i run RM2s. but i have a couple with the airholes drilled out to 5/64" and i find that too is that borderline area where you can choose how you want to vape it, depending on your coil build.
T
Depends on what you're after, but if it's flavor you want, RM3 is amazing. That's my vote for a dual coil on Rob's site (I have a modded Atomic, aka RM4 and don't much care for it). If you want clouds, RM4 or RM5 are probably the way to go (AFC ring and all)
My very favorite right now is the Madz Modz Reactor. Same great flavor/vapor as the RM3, but a bit easier to build on. You have to drill out a second air hole, but it is built for dual coils. I run .5ohms on both RM3 & Reactor on a mini 18490 batteries (amp limit on the AWs is 16.5, so I think very safe over .4ohms)