Same here. I got my full 200GB back a day or two ago, and the speed is back to my subscribed 8Mbps. Just got a call from someone from the cyber-backend division (or something like that), and they were apologetic about it, and explained that they have wiped all the data for this billing cycle.
As much as we complain on this forum, I think we should also give credit where it's due - Airtel has taken fairly swift corrective action, and giving the full data back for the billing cycle is a considerate thing to do. I am pretty happy with the outcome.