I remember having a roof-rack on my 850 Mini back in early 1964. I fitted it to move all sorts of things and the biggest was a complete single bed! The car wouldn't go much over 50 mph and it used a huge amount of petrol. Got some strange looks as well.
Roof racks are very useful with a Mini.
My wife once brought 4 Lotus-Cortina wheels from Hitchin to Oulton Park on that same roof rack, but on my 998 Cooper. I think it got to 65 mph and did around 20 mpg!
I must say I only used to fit it when I needed to move stuff as even empty it made a lot of wind noise ad knocked a bit off the max speed and used more petrol. Very useful, though.