The reason for stronger springs on the heavier GT "should" be to keep the same relationship between the sprung and unsprung weight.
The softer roadster springs will still be stiffer than when in MGB (same unsprung weight less sprung weight).
My preference would be to use softest available and contol roll with anti roll bars (have done this on my TCV8 used for sprints and hill climb).

Hi Peter
I used the 'down rated' springs (front and rear) as recommeded by NG together with the spax adjustable dampers. The front is fine but the rear is somewhat low after 5000 miles and two years. I have just replaced them with standard springs for the MGB rubber bumper roadster which give me more height (still 6 leaves).
It is still comfortable but at the moment a little high though it should soon settle. If not I can always add a plate between the axle and leaf to lower it but I doubt I will need to.
My exhaust used to catch every time I put the car in or out of the garage, that is why it was so important for me to have a bit more height.
For original details see my web site www.ngkitcar.co.uk, I will add the modification details in due course.
Chris