I know that rare Heatsink halves the cooldown time, and two rare heatsinks divide it by 4. But what about common and very rare heatsinks?
Basing on this analysis, these are the values of the Heatsink Mods reductions:
Very Rare: -70%
Every additional mod applies its reduction on the previous modified value, so 2 Common Heatsinks will reduce the cooldown to 64% of the base value (3.2 minutes), 3 Common Heatsinks to 51.2%(2.56 minutes) and 4 will reduce the cooldown to 40,96% of its base value(~2 minutes). The final values are probably rounded.