The fuel economy loss associated with upsizing wheels on the Prius is very common and occurs in all cases that I am aware of. I've tried dozens of different tires and more than a few wheels in an effort to better understand this loss and try to reduce it. Unfortunately you will always suffer the loss.
In an effort to back up the OP, the OE tire size for the 2004-2009 Prius is 185/65/15 and the OE tire is Goodyear Integrity with an 855 revs/mile. The average 215/45/17 grand touring tire has a revs/mile of 844. The tire is taller but even when correcting for tire size and its effect on the odometer, the loss is still vey evident. It becomes even worse when comparing the 17" tire to an ultra-LRR tire like the Energy Saver A/S.
I performed quite a bit of testing on my 2012 and I found the FE loss to be able 3mpg when going from the OE 15" tire (Yokohama S33D 195/65/15) with 829 revs/mile to a 215/45/17 Primacy MXM4 LRR tire with 844 revs/mile. When I tried the Michelin Energy Saver A/S 195/65/15 (832 revs/mile) my average mpg went up by by about 3+ vs. the OE 15" tire. When I switched back to the 17" tire I lost 6-8mpg. I backed this up with several more tire/wheel swaps.
Interestingly, my first set of 17" wheels weighed 15lbs which are 1lb lighter than my OE 15" wheels. When I switched to a set of 17" Lexus OE wheels the mpg hit stayed about the same despite the wheels weighing 24.25lbs! Obviously 17" tires weigh more than 15" tires (18lb avg. vs. 22lb avg.) but the weight difference doesn't seem to have a huge impact on mpg. I personally feel it has more to do with tire design. The 17" tires suffer more from hysteresis than the 15" tires.
Anyway, I just thought I'd share my opinion because I don't know anyone who has tried more wheel/tire swaps on the Prius than I have and I still can't solve this issue without throwing ridiculous amounts of money at the problem.
So for now, if you want max FE, stick with the OE 15" wheel and a set of Energy Saver A/S tires! The Ecopia EP422 is pretty good too. Maybe well get lucky like Europeans and get the new Ecopia EP001s.
