This. It sums it up pretty much, you would need either the flowrate of the loop to be much higher than water or have a much larger rad to benefit from it's high thermal conductivity. However as mercury is so heavy it will take one hell of a pump or much tuning and work on an electromagnetic pump to achieve that rate.
The think to look into would be a personalized ferrofluid or just a mercury based solution(and heterogeneity becomes a challenge here) or alloy with: high heat conductivity, high thermal capacity, low viscosity(to allow high flowrate) and possibly incorporating iron,cobalt or nickel to be used with an eleectromagnectic pump for better flow.
Basicly good luck finding the combination of budget and skills required to create a fluid like this and building the loop to run it. I know I havent got anything close to those skills