Does anyone have a Prius Plugin with a roofrack & box

The hit in efficiency should be similar to a regular gen3. If they lose 10% of their MPGs, you should lose about the same for MPGs and EV range.
Look into adding a hitch and using a rack/box on the rear. Shouldn't hurt your mileage and may even help a bit if its shaped and sized right.

 

The system is more efficient and more flexible to begin with. So, it isn't similar, nor does it matter anyway.

Why would recreation/vacation be of any efficiency concern? It's such a rare opportunity to get to escape. When you see a chance, take it. Don't worry about a hit when you are doing something that only happens on special occasion.

53 MPG is what I got the other day, carrying 2 kayaks without cockpit covers on top, for 37 miles using just a single charge.

That was totally worth it!

 

If it increases the drag by 10% (just theoretical) on a regular gen3, it'll increase the drag of a plug-in by 10%. That means the amount of energy required will be the same. So the mileage hit would be about the same after the plug-in's battery has run out.

 

Since it only takes a few minutes to put them on, there has never been a time that I've driven with them on empty.

It's just a quick clamp on each side.

You wouldn't want to keep them on even if there wasn't any MPG difference at all. They'd get gross, covered with squished bugs.

 

Here is my rack info:
Gen III Yakima Roof Rack | PriusChat

 

There could be a threshold where it is better to rent a boat (or what ever gear) at your destination then carry it on top of you car. Also, taking into account time, convenience, etc. Being in Utah we do some pretty long road trips to get to destinations.