Here is my idea: (current version 2.2J)
HeatShields efficiency related to heat level
and the HS not turn off, just idle or almost idle when overheated.
This is a calculation for better understanding.
Your mech HeatCapacity is 0/100 and your shield is 60% (ratio2dmg/1heat)
You get 200, 200 and 100 dmg in a row.
When 1st 200 hit, your HC 0/100 (100% cooled)
so your HS absorb 100% of your shield level (60%)
200:0.60=120 absorbed, 80 lost, generate 60 heat.
When 2nd 200 hit, your HC 60/100 (40% cooled)
so your HS absorb 40% of your shield level (24%)
200:0.24=48 absorbed, 152 lost, generate 24 heat.
When 100 hit, your HC 84/100 (16% cooled?!?)
so your HS absorb 16% of your shield level (9.6~10%)
100:0.10=10 absorbed, 90 lost, generate 5 heat.
Now your HC level "only" 89/100.
The weapon Heat Generation must be calculated before shield absorb damage like the Energy Discharge vs. Energy Shield.
Don't need automatic turn off for this kind of shields because they are idle (not absorb damage and not generate heat) if the mech runs out of heat capacity.
With this calculation you can make a new type of adaptive energy shield also.

WTF?!?...
...i think i drank too much coffe