Needless to say, full throttle should cause the engines to be *at*
full throttle.

Hmm, good point. I assume that with the engines off and engine hatches off I
could advance the throttles all the way and check to make sure that the
actual throttle levers on the engines are right up against their stops? Any
reason this would be a bad idea? Might flood the carbs but that's easy
enough to deal with.

You could also swap
the tach wires and see if the problem changes from one engine to the
other, thus determining if it is a tach problem or an engine(throttle)
problem.

Oh my God what a simple idea ... geez. Just switch my starboard tach over to
the port engine and if it gains the missing 250 rpm (or not) then I've got
my answer. With my luck it will read just different enough to inspire more
questions.

Thanks, that gave me a couple of avenues to try.