I've had roof bars and large roof boxes fitted to my last 4 cars but with the one prior to the PHEV I decided to also fit a tow bar so I could use a tow bar mount bike carrier.
I had the removable tow bar fitted to the PHEV for the same reason but this time, instead of fitting roof bars and the huge box I decided to buy an Erde 122 trailer with lockable hinged cover as this will be a lot more useful for carting things like lawn mower for the model flying field, and the like, around.
The trailer is parked on its rear end in the garage and I believe it takes up less room that the roof box I had (now filling my son's garage instead!).
I haven't driven the PHEV with a box on (others on here do) but my experience with previous rather lower cars was that they did make an impact on fuel consumption (and noise from above over a certain speed) - but not nearly as much as folk usually think.
I've only driven about 60 miles with the trailer on the back and to be honest I was continually worried (initially) that I'd left it behind at some point as there seemed to be no evidence of it's presence.
I suspect that it will have less impact on fuel consumption that the box on top of the fairly high PHEV and I'll be able to see something through the sun roof when then the clouds clear. Cleaning (rare) is easier without the box on (I tended to just leave it on the car unless I was booked into a low headroom car park somewhere).
The trailer can be fitted with bike racks too in the event that all of my unused camping gear gets out of the house and I want to take the bikes as well so personally I'm still happy with my decision.
Not a lot of help, but my (current) personal thoughts.
JimB