From what I have been reading the built in probe is as good as the old one. Everytime a bios upgrade happens the temps change aswell!!
That probe is as good as the bios makers make it. I don't like the idea of using a actual computer in the first place to test. I was going to setup a test bed that would consist of pelts with known wattages/voltages/consistant temps and you can put your own (accurate) probes in. If you can get the heat source to remain constant and consistant you will get better results. Also you will rule out any install errors that may happen.
I maybe way off base but that is how I would test water blocks for accuracy.