Sensitivity is one thing, backoff threshold is something completely different. More sensitive AP means that it actually hears weak signals and can make competent decision backoff or not. If sensitivity is not so good, AP just can't hear this signals and takes no actions. It also means that level of own noise is reduced, so for normal and low signal levels SNR is a bit improved.
I don't know how exactly backoff policy is optimized, but it is obviously that it is done in Ruckus (and probably all other enterprise vendors do it too).
I have seen multiple installations near busy wi-fi networks (mostly hotels), where level from neighbor hotel network was about -75 - 80 db, no other interference, and SOHO equipment just didn't work (Linksys, Ubiquity, Mikrotik) even when client is nearby (5 m).
We observed 100% signal level, packet loss about 20% and ping times 50 - 1000 ms. So nothing worked. I suppose reason is that AP hears hotel network traffic all the time, and as a result has almost no airtime, because hotel network is heavily loaded and ignores AP frames (or doesn't hear them at all).
Just replacing AP by Ruckus resolved this problems.