It's true and documented in several places. For example, the Japanese govt. testing site lists it. It's not a big penalty though. There are a lot of very happy touring owners in the US. The European car is only a "touring" model.
So you might get, say, 50 MPG with a standard model, but "only" 47 MPG with a touring. As I said, not a big hit. A headwind would have a greater effect. Not inflating your tires over the door placard specs. would have a greater effect.

The cause is the larger tires. Better traction but higher rolling resistance. A fair trade off.

If you really want a touring model, I recommend you get one. Test drive both to see the differences. If you can't tell, get the standard model. If you like the handling of the touring over the standard, get the touring.

Pearl is a
2007 Driftwood Pearl Prius
Package "B" (everything but leather, nav, and rear camera)

I agree with what David said.

Test drive both models, and if you like the handling of the Touring, get it. Otherwise, stick with the standard model.