Does klonopin (clonazepam) (or any benzo) ever really expire, i.E. Lose potency over time? In other words is it just as effective 20 years past expiration date?

20 years? 20 years is way too long to rely on the potency of the listed dose on the bottle. Some medicines can still be active a year after their expiration date, but i wouldn't trust if after that. Lots happens with heat. Light, moisture, etc. For a more detailed answer, please ask your pharmacist.