I'm using CrashPlan+ Family Unlimited (it's a home NAS!); I also bought a separate CrashPlan+ license when I first signed up. I don't have more than 1TB backed up specifically to CrashPlan as yet though. A big chunk of that NAS is for future growth, and a lot of it is a playground for experimenting with serving up media that we either have physical copies of or is of low replacement value.
I do see CrashPlan updating at a leisurely rate, but that's OK with me, as my upload bandwidth is limited. I'd be interested to see if it completely chokes past a certain point, like you say it does for you. But it's not yet at the point where I need to worry about it.
Are you using Crashplan+ or any of the Pro/Enterprise plans? What sort of transfer rates do you get? And were you able to complete a full backup of that 11x4T array?