I think you have something wrong... it isn't going to stream anything from your time capsule drive. Your purchases will come from the iTunes Store, but if you put your iTunes Library on your time capsule drive, it is still going to go Time Capsule (aka an internal AirDisk) -> Computer -> Airport Extreme (built into Time Capsule enclosure) -> AppleTV because it has to stream via an open iTunes library.
I'm pretty sure Kevin, who is an engineer on the AppleTV team by the looks of his website knows a lot more about what he's talking about than you.
That makes no sense to add a hop into your streaming. Both for latency sake and simplicity sake. Why have my Mac on to stream from a disk independent from my Mac? If this is the case I'll continue to use my AppleTV as I have. I stream directly from my AirDisk (via hacks).