If you decide to do this, I would take the ESC out of the case before you start drilling. :wink:

Personally, this just lets in more dirt/dust and shouldn't be needed, especially if using a UBEC. And if running the MM that hard where the bottom layer is overheating, maybe the MM isn't the right ESC to use. IMO, extending the pins that connect both PCBs (to create a larger gap) and then adding a heatspreader would be better.

Some extra dirt/dust does get in there, but It hasn't caused any trouble so far. The temp difference is very noticeable. If you have the fan blowing through the holes and across the top of the heatsink, it stays nice and cool.
http://www.hitnget.com/ebay/electronics.jpg

My fans came today. I might order a 12t to try, but I'm not sure I want to slow it down any more. I built this thing for speed. Doesn't matter that it will still be faster than nitro, as it's about 200 miles to the nearest good track, so I won't be racing it. If the MM won't handle the current for the higher speeds, I'll just have to live with it until something better comes along. Ultimately, I would like to gear up a few teeth for even more speed. Maybe we will see a new esc soon.

Thanks for sharing the pic.