Luscious is a shop like no other. It is worth the drive. I would recommend booking an appointment with them before going any farther. Also, I would recommend you have them service your Prii from now on.
A/C..... I can be averaging 48-50mpg over 200+ miles and my wife has it for 3hrs during the summer and when I get it back, 37-41mpg
Another plug for Luscious. I take my 2006 Prius there and am a super-satisfied customer. They are honest, super-nice, and smart. They are also very knowledgable and experienced with hybrids. Since SF mandated that all its taxis be hybrids, they seem to have become be the go-to garage for taxis. I wouldn't recommend driving in from Castro Valley for something straightforward, but if your mileage has really progressively declined, and is now consistently 32mph, I think it's worth their diagnostic skills. I drive in hilly SF, and I've never gone that low. Yes, it's under warranty--so if it turns out to need a new battery, you'll take it back to the dealer. Website has an informative blog. Luscious Garage | Hybrid Specialists There's a link to email them on the home page. If you email Carolyn, the owner, she may be able to give you an idea of what might be going on. Plus, if you go, you'll be able to say that your garage was featured in the New York Times.
Usually a good sign. If the local taxi drivers trust a garage, then it's for good reason. No messing about, quick diagnostics, they know what they're doing etc.