Thats what I had determined from my research as well. Have you measure the difference in heat transfer without solder mask? I had planed on over clocking with a 14mhz oscillator but ran into a brick wall with the cooling. I tried a variety of cooling solutions. At one point I removed the head spreader and insulation sheet, put artic silver directly on the pcb and bolted it to a intel cpu cooler. The heat transfer though the pcb  was the issue, the cooler stayed cool but the top pcb layer around the chip was hot. If you say that it can be improved by removing the solder mask the I may need to try an remove it. Acetone will not work its used on boards to clean up hand soldered areas so the solder mask is resistant to it. If it's acrylic based standard off the shelf paint stripper can be applied with a Q-tip or brush. Sand paper work well also. The schematic and data sheet for the voltage regulator is posted in one of the first posts. Its a standard voltage regulator that is set by a voltage divider and compared to an internal reference. Let me know if you get any farther with this. I had all but given up on the idea, but I may have another go with it if the temps can be brought down at stock frequency.