Well that's not encouraging to hear!
Do I at least understand correctly that the high speeds on this shutter result from the pallet being disengaged from the low speed escapement, but with the escapement gears still engaged? That's what it looked like to me. Also, can you confirm - is the high speed cam designed to come into play ONLY at the highest speed?
As for springs, I have had good luck with very simple shutters bending or re-making them to restore (or even increase) speed. But it could be trickier with a "real" shutter since it is not just a single speed I am trying to get right.