two little wishes:
a very little but (for me) very helpful thing would be to have the small material preview updatet too when you have rendered a bigger preview.
for now you have to update evry material twice if you work with a bigger preview.
besides this it would be nice if there were a one button solution to update all previews at once. or an auto update running in background on one cpu core (or a definable number) every time you change a material or something like this...