The temperature of the stagnation point on the leading edge might be TAT, but that isn't where you are storing your fuel. The temperature of the air over the wing skin is a function of SAT and aerodynamic lift, and in particular is COLDER than SAT (which itself is of course a lot colder than TAT).
I disagree with that, CJ Driver. You're ignoring the boundary layer.
The problem is that while the air at the outer edge of the boundary layer is indeed travelling at the freestream velocity (or faster if the surface is lifting), the viscosity of the air means that the velocity (with respect to the surface) falls off to close to zero at the skin.
Because of conduction through the air, it's not as simple as the skin acting like a stagnation point and the skin temperature being equal to the TAT. The skin temperature (on the assumption of a skin that is neither a source nor sink of heat) is SAT + r*(TAT-SAT) where r is a
temperature recovery factor. r depends on the thermodynamic properties of air and is about 0.85. So the temperature rise, in effect due to skin friction, is 0.85 times the ram rise. It's this skin temperature that you're likely to be measuring with a simple OAT indicator, unless it's deliberately set up to measure TAT. Thus the skin temp is just 15% of the ram rise less than the TAT.
Moving to the lifting wing case, the pressures, and therefore temperatures, at the outer edge of the boundary layer do indeed decrease over both surfaces, particularly the upper surface. If you make some assumptions about the likely ratio of the peak pressure coefficient to the mean pressure coefficient, you can see that the temperature reduction is proportional to wing loading, and has some dependence on the ambient pressure.
I ran the numbers once and deduced that for typical light aircraft wing loadings, points on the skin can be 1 degC below the indicated OAT, and for big jets it can be as much as 10 degC. I'd hope that this is consistent with your FTPP.