new tires = 20 mpg drop?!

Why do you feel you need an alignment ? It's not necessary to align a vehicle to replace tires.
It's needed if the tires are wearing improperly. I've put 80K on my last Prius and never needed
an alignment. Keep it out of the potholes and don't bounce it off curbs and you'll be fine.

 

I would imagine different new tires will have different dimensions changing the alignment. I just got the car a couple days ago so maybe Ill wait and see how the MPG is with these first.

 

Tires won't change the mechanical alignment of the vehicle. I changed my stock tires and wheels with new rims and tires of a different size. Hasn't change the alignment one bit.

I went from a 175/65/15 to a 185/65/15 and a slightly higher off-set rim. Tires run even and flat at 42 PSI. The car steers the same, the wheel is still centered and the it doesn't pull Nothings changed except the looks of the car.
I did it after 1K miles, so I could compare the change with the stock setup. The only thing that's changed is the ride quality and it looks 1000 time better without those hideous plastic wheel covers.

 

The best thing about the OP's experience: it didn't happen to me.
But seriously that is weird, lesson learned. It seems the fuel savings of LRR tires would really add up over the lifetime to adjust for any savings of buying non LRR tires. I'll be sticking with my OEM Michelin Energy Savers when the originals wear out.

 

Rolling resistance coefficients (RRC) range from about 0.006 to 0.015 with the higher end representing extreme performance tires. Realistically, RRC range over about 30%for tires of the same genre. Couple this with rolling resistance being perhaps 10% of a typical vehicle's loss and LRR tires should provide about a 3% increase in MPG. Now things could be a bit different for a Prius because power-plant losses are less but how much? Perhaps there is a 6% difference.

So something else is going on here. Have you done a full-tank run yet? Did the tire retailer leave you vehicle in READY mode while on his lot, thus consuming gasoline with no miles added?