Like I said it's because your bios is enabling cooling for the CPU, (reducing the power to it when it's not running full task).
You'll find most people that get only 5ºC difference are not running with this cooling or are running a CPU (like PIII) that doesn't generate anywhere near as much heat.
If you would prefer the temp difference nearer to 5ºC between idle & full load, then use a newer KT7a bios and it will run close to your max load temps all the time, other than active cooling there is no way you will get max load temps on your set-up that are 5ºC above your idle or water temps. As
Redleader said with greater flow you
MIGHT get better temps but it will be a few degrees at most, not 20ºC
Personally I'd join the flat earth society before I would believe someone has a CPU like mine running with 5ºC difference, between Idle and max when CPU idle instructions are enabled
