I've been using your stunt system (pretty close, a little bit more freeform) and its gone fine so far. I'm still having to coax my players to using them, but they are starting to get the hang of them. Generally I've been using the hard DCs, and I'll slap on a +2/-2 on certain skills depending on the conditions.
One idea you might try for extreme stunt to simplify it is to make the bonus +3 and the penalty -5, with no change in the DC. Quicker and easier to calculate, but still makes for a bigger gamble on the player's part.
One idea you might try for extreme stunt to simplify it is to make the bonus +3 and the penalty -5, with no change in the DC. Quicker and easier to calculate, but still makes for a bigger gamble on the player's part.