I don't think coolant is the problem, the problem is how coolant works with double heatsinks. It is literally a tap of the coolant with any Clan 'Mech, but with IS you have to hold down C for a bit to get any substantial heat drop. I think it is sort of janky that Clan weapons create more heat, but due to how heat works they really don't have to worry about it, most Clan crap doesn't even really need coolant to be effective, their heatsinks are just that damned good.
I don't think IS lasers need a buff really either, but they are subpar with how heat works.