That was my experience--down to the "torrential rain" thing, not to mention a couple decent winters.
Mine's a company car; when they changed out the OEM tires a few weeks ago and put on some God-knows-what things, my mileage dropped like a rock. Hint: the Prius is a carefully engineered system, down to the tires. Change out anything, including the tires, and you break the system. Want to keep the mileage up? Keep with the Integritys (or similar tires designed for high fuel efficiency using low rolling resistance design).