FEX or specific, I believe, to Nexus series. Did you mean IAs?
On L3 switches, routing isn't the bottleneck it once was. Something like a Catalyst 68xx generally has sufficient resources to handle lots of networks. So, big networks, like your /22 don't really offer much in the way of any benefit. They can, though, tie up a large address block, which might have a low host usage. Also, a larger network (again like a /22) exposes host to the broadcast scalability issue. Lastly, if your ports are data/voice, your VoIP devices are exposed to physical port traffic, such as broadcasts, from the data VLAN too (although the VoIP phone "logically" will ignore it).
