TDP is not the max power draw of the chip, it's a number used to let OEMs build sufficient cooling systems. You can probably find some apples to apples max power draw benchmarks out there, but I would guess that the difference between the two while mining is minimal, a few watts at most.
I realize that that TDP refers not to the max power draw, but to the amount of heat a cooling system can dissipate. However, those numbers are there for a reason. If I single core chip is rated for use with a cooling system that can only handle 35W of heat, and another dual core chip is rated for 65W, you can expect it use about 2x the power at max load. It might not be exactly 2x at idle, but I'm guessing maybe 1.5x?