As pointed out in this threat "To all those waiting for an SP3 with i7 for HD5000 and gaming you may be disappointed" on Tabletpcreview(cant link to it as WPcentral sees it as spam).
It seems the system is over-throttling, as it keeps the core temperature at 60 degrees Celsius with prolonged CPU/GPU activity like gaming. Compared to the Surface Pro 2, that keeps the system throttled at 80 degrees.
So in theory MS could fix this "performance problem" by updating firmware, since there is no point in keeping the system at 60 degrees when it can easily handle 80-90 degrees, leaving a lot of headroom for less throttling!
So im crossing my fingers that this is not a termal issue but a software/firmware issue, that can be fixed!
That being said, I am looking for portable and powerful, not gaming, so im keeping my preorder (Still 2 months of waiting though).