WC-would this be more efficient

[Dug]

larger reservoirs slower flow through radiator. In addition, won't colder water (down to 4C) tend to stay in reservoir2.

[Aesik]

How is having the larger reservoirs going to slow down flow? The net flow rate through the sytem will still be the same if you are using a swimming pool for a reservoir. Everything the pump is pumping still has to flow through the radiator at the same speed it's flowing through the pump and through the waterblock.

I'm not sure what you mean by colder water in the lower reservoir, please elaborate. If you mean the water in res 2 will be colder than res 1, it should be because it has passed through the radiator. If res 2 is sufficiently large, and the forces of the incoming water from the radiator are not too large, the bottom of the res would have very slightly cooler water than the top.

[Dug]

I see now it won't work. I was trying to use the fact that 4C water is most dense and sinks to the bottom.