Thanks for your reply.
1/ First of all, about the heatsink : won't it be a problem with daughterboards pluged on the 40p connector (a.k.a raspberry hats) ?
2/ About *ubuntu, please take care on what is provided out of the box by Ubuntu : most of the time, it's only a collection of shits and craps ! As an example, Epiphany won't work at all. Another example is provided PCManFM lacks network capabilities. Idem with LXDE that doesn't start only due to a stupid misconfiguration, etc, etc, etc ...
Personnaly, I gave up for a long time with Ubuntu as I was feedup to deal with brain damaged choices of instable softwares whereas good one exists. I can't imagine what will be the result for your board (and Linux at all), if you send your board with Lubuntu to a journalist that is not aware of Ubuntu "features". Please consider about more decent distribution.
3/ Speaking about that : where can I find the source code of GPU stuffs ? If I want to install Gentoo, I can't use (well, I can, but it's tricky) debian packages.