The point is that when you lower the tdp in afterburner, the termal limit is also reduced. So 70% tdp will give a termal limit of 70 degrees. This meens that your gpu starts to trottle @ 70, but most gpu's work fine @ 80.
So 70% tdp, 80 degree term limit. give more hash than default 70/70.
To check the cards max termal limit, check the link button and slide tdp to 100%. You shouldn't exceed the cards max termal limit. My g1 gaming cards have a limit of 83 degrees.