Listen Interval indicates how long the station will be "sleeping" without listen to any Beacon transmitted by the AP's when the station enter in power save mode, those beacons includes the DTIM information. The relevant point here is that if we don't tuned properly these 2 attributes it could happen that when the AP transmit the DTIM letting the Station know that is some information in the buffers to be transmitted the Station could be not listening because the Listen Interval is so high for instance.
802.11v allow the AP's to suggest the clients to roam to a specific AP or give a list of preferred AP's to roam. At the end the client decide when to Roam and where to roam.
Upgrade any WIFI network to a new standard always generate higher expectations from the end users however as WIFI Professionals we need to be careful with the impact of use a higher modulation technique in the high rate effective coverage, the general rule is higher data rates demands higher modulation techniques and Higher Modulation techniques demands smaller cell sizes because the higher modulation techniques are very sensible to noise and data errors.