I'm doing a little exploratory C coding in AIX, with the intention of
getting system and process performance metrics, and I'm trying to find
good reference docs on AIX system calls.

I've found the "Technical Reference: Base Operating System and
Extensions" documents, which are a great reference only after you know
what subroutine you're looking for. Are there other docs that would
categorize the system calls by section (file system calls, CPU calls,
something like that)?

As I mentioned, I've been through the technical references in the
pSeries Information Center, but the docs there were either to general
(General Programming Concepts) or too specific (Tech Ref: BOS and
Extensions).