I don't think there is any hard data here about restoring your DVD to original firmware and not getting banned.
There are those that say MS can detect if you have played back up games offline. Some say yes, others say no. Right now, it is really hard to tell. Someone could have been flagged for a ban, reverted to original and were banned when they logged in. Others could have reverted to original without being flagged and caught and banned.
There is no way to tell when a person is flagged or how long it is between flagging and banning.
This is what I have gathered from my readings here.
If you want to revert to original and try it, go ahead. There isn't any guarantee you will not be banned. The only safe way to play on LIVE, in my opinion, is to have a non modded console and you play only originals with it.