Just a note regarding expired yeast. I had some WLP550 that Expired on Feb-11. I was getting a little concerned since I've never gone that far past the expiration date. On July 3 I decided to use it in a 5 gallon batch. I made a 1.2 liter starter. I planned to wait longer than my normal 24 hours before cold crashing the starter, but I had something come up. I couldn't wait. At 24 hours the starter was showing slight signs of activity, though not as much as I would anticipate from fresh yeast.
After cold crashing and decanting most of the liquid, I pitched the yeast. I expected some lag. However, after 8 hours fermentation is going strong. So nearly 5 months past expiration -- with a normal starter -- things look good. The beer OG was 1.056. I'll post the FG in a couple of weeks.
Finally, I made several slants from the expired yeast. Even the slants are showing strong yeast growth after 48 hours.