Howard1, I did a test on a motor with different spring combinations on a Cobalt motor. Here's what I found:
with Green/Green springs average efficiency as 27%
with Red/Green springs average efficiency as 26%
with Red/Red springs average efficiency as 26%
with Purple/Purple springs average efficiency as 24%
Max efficiency followed the average curve by about the same percentage - from a low of 46% max to a high of 49% max.
Basically, the lighter springs gave better efficinecy and lower overall amp draw and the heavier springs had the opposite effect.