The cooling system in the Mini has used the same basic style radiator since it's begining in 1959 up until the introduction of the twin point injected cars when the radiator was moved to the front of the car.  The original Mini was powered by an 850cc engine and designed for the little back roads of the UK.  The cooling systems demands were nominal.  As engine capacity and performance demands increased so did the need for more efficient cooling.  All of this had to be accomplished within the limited design space allowed for the side mounted radiators with production cost being a key factor. (even Plastic header tanks can be found on late model Minis!)

With the closing of all real facotry capacities, the aftermarket industry came to the rescue first with the introduction of modern production technique for manufacturing and then with improved flow and performance. 

Having built Minis for the last 35+ years and installed them in both street and race cars, I thought I would share by experience. While the chart below is based on the use of the early 2 peice radiator, the one piece shroud simply make it a little easier but has no effect on cooling.