I think you're on the right track for this test. If the test only looks at HC, CO, and Lambda it sounds like you should see favorable results. Don't let out the secret of the DIY "sniffer" test. Although it isn't an expensive or complex diagnostic tool, I've used it myself in a pinch.

You might want to reconsider converter position or exhaust pipe configuration before your next test. You may be able to return to your previous timing settings to improve mileage if you can get more engine heat to the converter. Many newer vehicles position the converter immediately after the manifold. This maximizes heat at the converter. I've seen custom systems built which place a pipe inside a pipe to reduce heat loss. Maybe this is why some GM trucks have a similar configuration on the Y pipe pre-cat.