Do you have a temp gun handy by chance? Because thermometers are probably not going to get the most accurate reading as far as surface temperatures go.

Is there any "lift" at all from the probe thats preventing the heatmat from resting flat against the bottom of the cage around the probe area?

I'm also using an ultratherm(6x11 and 7 watts) for my AP cage and I have no problem getting my hot spot to 89-91. So I'm pretty sure yours is more than efficient.