In practice, you would not dream of descending that far. The most I ever came down was 4000'. You cannot disregard setting off the eventual climb back up to altitude from your theoretical savings of descending.
It is not as uncommon a practice as you may think, westbound over the Pacific, approaching Japan. I have seen 200Kt+ headwinds at altitude with no significant drop-off until 6,000-8,000' below planned cruising altitude.
Personally I have used the tactic to "duck under" the jet stream a few times, and other pilots in the company have reported the same. The key is to use and understand the wind trade charts in your Performance Handbook (or wherever you find the charts for your operation), and have a good confidence level on the actual winds at the planned lower altitude.