Don't worry, I know the difference between lag and stuttering. Problem with SC2 is that my ulv HD4000 will constantly vary clocks and thus FPS, especially when scrolling. The difference between my experience and yours is probably those 300~450 Mhz lower clocks.
Not arguing against that, my point was the rather stupid naming convention of Intel. They call everything HD4000 even though the performance can vary by 40% (ulv 700~800 mhz effective, qm 1300 mhz).
Stacked DRAM is very, very close to going into production. It misses Kaveri's launch window by a bit, but who knows. I would say that GDDR5(m) would be a good solution. Standard DDR3 obviously isn't cutting it.
Talking about my own experience here. It can reach 1 Ghz depending on the game (and in benchmarks with zero cpu load), but it can not sustain it. Most of the time its at 750 Mhz, jumping up and down slightly.
The problem with that is that a 256 bit interface would mean 4 channels of memory... and when you're building a laptop, saturating 4 memory slots is going to be not only expensive, but is going to add to the size of the computer. Larger size and laptops do not agree with each other.
