there's no air on top of the heat spreader, the keyboard sits directly on it. Ideally anyway the keyboard is directly conducting heat off the heatspreader, this is extremely more efficient than transferring the heat to the air and the air to the keyboard.
also, i would just assume people are taking their before and after type measurements with the bottom un-obstructed.
With an ambient temp of ~85F throttling the gpu and cpu of the mini resulted in acpi temps of 74C fairly quickly. This causes the gpu to transparently throttle down (possibly even the cpu to do so, though i think it would report that via cpufreq and no such thing was witnessed).
Fixing the thermal pads (which were installed by retarded blind chimps obviously) had the effect of cooling idle temps by about 5-8C and kept the cpu from ramping to 72C for a lot longer.
The fact that it still reaches 70+C means that the problem isn't entirely in the movement of heat to the heat spreader via the thermal pads (which are crap) but in removing the heat from the heat spreader to outside of the laptop. This means the conduction of heat to the back of the keyboard, as there is no ventilation above the heat spreader. Making sure the conduction is optimal (grease/paste) can only help. But i think ventilation is needed to remove the stagnant air insulating the inside of the netbook.
The bottom door can use some serious enhancing when it comes to ventilation, I already outlined my plan for that. It should be fairly straightforward and look fairly stock aside from it being metal.
The side vent can also be improved, there is a RF shield behind it that seriously impedes air flow. I simply bent it back and forth until it broke off at the bottom crease.
The other idea is destructive to the mini, so it's obviously the last one i would attempt. Drilling holes into the cross member above where the keyboard sits. This would provide a direct path for air to ventilate around the heat spreader, while not being really visible nor being an easy way for dust and what not to fall into the mini.
Other destructive ideas include removing the ethernet port and/or a usb port to further open up vents on the side.