aa is only meant to fill in the microscopic gaps between joints. like in between the led slug and heatsink, or in between a heatsink and flashlight body.
using a disc of aa like what you're talking about would offer very little thermal transfer, compared to mounting the emitter directly onto the heatsink.
the smaller the aa joint the better, so if you had a joint thick enough to fill the hole in the esink, it would transfer heat horribly.
i can't remember the numbers (someone might try googling it..?), but if you do look it up, you'll find that the thermal transfer of a solid piece of aluminum is tens of times (hundreds?) better than a thin joint with arctic alumina, so you can only imagine how horrible a disc of the stuff would be.