I agree that in theory, it shouldn't be a big improvement, but anecdotally I've noticed a pretty big difference even at far distances when I've replaced N equipment with AC. I recently replaced an 802.11n 7782 with a T300. At far distances, the 7782 was delivering 70-80mbit to a 2-stream device, which is well below 802.11n's capabilities. I thought replacing that with a T300 wouldn't do much good since T300's main advantages were 256QAM and 80MHz channels, and neither of those should make a big difference.... But now I'm seeing 130-140mbit at the same distance.
BTW, note that Ruckus AP's are pretty smart about picking AP -> client TX rates. All rates and widths are considered. At far distances you'll see the AP automatically pick a 40MHz or 20MHz MCS rate over the 80MHz one. If you dump the support data and look at the Beamflex debug data, you can see that MCS/width candidates in addition to beamflex antenna pattern candidates are all ranked together.