While not scientific I have been playing around with Straight Talk (at&t) to determine if I can be happy with their service and discovered a few interesting things.
The proxy makes a huge difference, with the proxy I was throttled three days after using Pandora in my car and watching a few YouTube clips (approx 100Mbs).
Without the proxy I used more DATA (2.5GBs) streaming Pandora all month long in my car, watching YouTube clips and using my Slingbox and I was not throttled once.
Four months in and it seems that the proxy is the cause of throttling for minor stuff. My guess is using large amounts of DATA (6-7GBs)would probably still get you throttled.
Dan