So, for example, if I have a VPN road warrior set up, and I'm somewhere where the network is 10.0.0.1/24 but the VPN tunnel network happens to be 10.0.0.1/30? Changing the client network address ...

Subnetting doesn't have anything to do with real IPs. It affects the routing on your network. In that sense, yes it could increase internal network performance, depending on how many computers you ...