Hmmm... two different beasts.
For 5-stand, it would be fine. I often shoot 5-stand throughout the hunting season, merely to keep in practice. When doing so, I will frequently use a 20ga, and my scores don't seem to suffer much (maybe 1 or 2 fewer hits per 25 on average).
Bunker is THE game which I would dearly love to try. As I understand it, the clays tend to come out faster than regular trap, and at a variety of heights. A rapid second shot is often the key to success. For this particular game, I have a feeling that a 12ga would hold a reasonable advantage over a 20ga for most shooters.
Frankly, if it's the gun you've got, and you are comfortable with it, I'd try it first. If you can shoot with a friend (and borrow their 12ga for a few rounds), then that might be the best way to find out.
Then again, if you're looking for an excuse to buy a new gun, who am I to discourage you?