When do you reach the point of "OK, this has too many tests"?

My attitude to optimisation and benchmarking is neatly summarised by these two quotes:

"Premature optimization is the root of all evil in programming."
-- C.A.R. Hoare

Rules of Optimisation:
Rule 1: Don't do it.
Rule 2 (for experts only): Don't do it yet.
-- M.A. Jackson

Basically, CPU cycles are cheap. It's much more important to write clear elegant code that you can come back to in six months and still understand.

Most of the time you'll find that the most obvious clear code is more than fast enough. I only reach for Benchmark in the very few cases when this isn't the case.