I actually never considered the jarring action of bottoming out on a driver's vision, but it makes sense. With regard to heat, it's definitely a factor. In the Sauber "split car" video, Perez rather sheepishly admits that his ass gets a bit toasty during a race. But, Sauber haven't done anything extraordinary to mitigate that heat. So, one can assume that it's to be expected. If, on the other hand, the RB8 is generating enough friction with the tea-tray to actually slow the car and necessitate additional cooling, I'm pretty sure the plank would then be worn away more than is allowed.
@ FrukostScones (What does that mean anyway?)
Downforce always equals drag, no matter what. Some methods for generating downforce inflict lighter drag penalties than others, such as ground-effect, but the penalty is always there. The RB8 generates so much downforce through every available medium that it never wants for speed through fast corners. But, the car will never set the speed trap on fire as a result. That's ok, because lap time is all that really counts. (Unless, of course, one is being overtaken at the end of a DRS straight, which is why Vettel goes hard for P1 in qualifying and tries to start the race like a bat out of hell to avoid falling to within 1 second of any rivals behind him.)
One second thought, to go along with the original question, maybe the splitter is being run in such a way to make contact with the ground so as to stall the floor to relieve the car of some drag along straights where downforce/drag is an unwanted hindrance. The active-suspension era Williams chassis was lifted on straights for this very reason.
As they say, there's more than one way to skin a cat.