1.You only lose 10%, not 20%. You typed twenty. It might have been a typo.
Did that change? I could have sworn it was 20%
So yeah, for everyone except Gorgons and Non-Human Monks it's an improvement regardless of your level.
2. You aren't counting the less regen monsters get from burning.
Irrelevant; that applies whether you have the sigil or not.
3. When you're using a regen strategy, you'll actually get to loose more fireballs at the opponent.
Actually this is incorrect; because your relative health regeneration is higher
due to the sigils, you'll actually get fewer
fireballs per attack than without it.