Is there a more efficient way to get Postfix to talk to SpamAssassin without the use of shell scripts as content filters?

Yes. Using spamd would be the best way to accomplish this.

Forking new processes is very low cost. This is what Unix/Linux is designed to do. Building a new SpamAssassin process for every email can be fairly expensive as it is a heavy weight process. For this reason, spamd was created. It will fork off a few processes and keep them around to filter your email. The pool size is dynamic and tunable.