@lynchfourtwenty - Have you tried using the stock o2 sensor and using the wideband just for monitoring purposes?
The stock O2 sensor will respond faster when making corrections and might help you gain some of the MPG back.
What is funny is that my car has been running on the exact same speed density tune year round and I haven't seen any of the huge fluctuations that you are talking about.
I am more than willing to help you out with getting it dialed in, I just need a few logs and your TCD to look over.