Yes fungus I have read somewhere that what you said is no longer true in the recent versions of arduinos because of changes in the power circuit (can't find the post, but it was from a arduino team member).
I like to take care of my things, Arduino Mega require at least 7.5V, so be it, I will feed it with 8V so it stay cool 
I agree it may be a little "overkill" to not use the 5V and 3.3V from arduino, but it is more convenient for my project.
Thank you anyway!
Oh and today I received my Arduino and touchscreen! Let the fun begins!