I've had admin on an account that did about half a million USD on AWS monthly in the past; we cut it down a lot by expanding physically and going in hard on reserved instances, but my view into things started around there. I'm unsure if we ever attempted to negotiate a SLA, but we paid for premium support just like you (despite having a dedicated, and great, sales rep) and got kickback credits from the general SLA policy[0] just like you. Nothing special.

If anybody has a special deal, it's probably Netflix. I suspect even half a million monthly is a smaller account to Amazon.

[0]: https://aws.amazon.com/ec2/sla/historical/ specifically, not the current (better) one