When we rented a Prius for a trip between Albuquerque and the Grand Canyon, we found that the combination of the two power plants meant that our cruise control REALLY worked -- as in we kept having to go off the cruise control when going up hills because we didn't want to run up the tailpipe of people driving a gas-only engine.

I figured the issue was that the gas engine would come on and then, as the car would slow down, the battery would supplement the engine, and we tended to absolutely fly up those hills at the same speed as we held on the flats.

No way was that car underpowered.

Now maybe that's a different issue that purely acceleration--though we found the Prius plenty peppy.

Save the world: Reduce greenhouse gas emissions 5% annually.

Would it be more efficient to tow a small trailer behind the Prius than to put that bulky carrier on the roof?

Your carrier is not any bigger than some of the small trailers pulled by motorcycles.  

So, what does your owner's manual state about towing?  Can it be done?  

I know, I'd prefer not to have two more wheels back there, but leaving out preferences, what is the feasibility?  

As usual, jabailo is full of **it.  Yes, there are big differences between subsonic, transonic, and  supersonic aerodynamics.  But these differences are a result of the compressibility of air.  As the vehicle travels closer to the speed of sound (i.e. the maximum rate of movement of the air molecules), the effective compressibility of the air changes and shockwaves form (thus sonic booms).  But the speed that a car travels are all firmly in the subsonic realm, whether it's 20 mph or 80 mph.

And, yes, I are a rocket scientist.  (At least, I used to be.)