Hehe, a bigger tubing has no effect on the temperature. You might have a better flow but it is useless.
This site (german) explains it in a very detailed and scientific way.
I will (try to) translate an important part of this text:
"The influence of the flow rate on the cooling performance is often
excessively overrated. In forums is speculated; one often believes what one wants to believe. Manufacturers, that set on xtralarge (10mm of hoses and more), defend these systems for in the long run with understandable reasons but incomprehensible arguments and pseudo formulas."
Some diagrams explain the influence of flow rate on the cooling performance and the most time you see no differnce whatever flow rate is used.
Only with less than 20l/h you will be able to see a noticeable differnce.
Well, I don't want to have a hosepipe in my system ;D ;D