Performance gain with MultiSubnetFailover for single subnet
When MultiSubnetFailover=True is set, SQL Server Native Client will aggressively retry the TCP connection. 

So in 


a multi-subnet scenario, it will attempt connections in parallel, which you already know.
a single subnet scenario, it will aggressively retry the TCP connection faster than the OS default TCP retransmit interval, so your reconnect time after an AG failover will be improved.