I have no idea about how games are constructed (well, not since the BBC Micro at any rate). So is traversing a TCP stack normal, or can a faster client/server interconnect be implemented for a purely single-player game (S42)?
Well, I'm not a game developer (unless hobby projects count), but I have been at various times in my career a p2p protocol developer, general network developer, OS developer, distributed application developer, but never on Windows, so pinch of salt. In answer to your question, yes, you can use a lighter transport than TCP where the client and server are both on the same machine. This can give performance advantages where you have high throughput. However, whereas on *nix the semantics of using TCP sockets vs local sockets are very similar, on Windows, the local IPC mechanisms are somewhat different to the network ones. I would not be surprised if Windows developers would forgo the efficiency of a local transport if it meant additional work to abstract the differences between TCP and COM or named pipes.
In any case, independently of the transport used, if the (local) server is implemented inefficiently and causes the client to wait on it, the game will not perform well (locally or in multiplayer mode).