I've got 12 GB of RAM, and never seen a problem. I can't give you exact figures, but I don't think that ECC RAM has much point unless you've got some mission-critical setup, in which case you'd go for more expensive server / workstation hardware anyway most likely.
But as for it not being on the support list, I don't see that it is going to be a problem. The memory controller is on the CPU anyway. And memory incompatibility errors are very rare in my (limited) experience, as long as it is the right type (DDR3, unbuffered or registered, non-ECC or ECC).