I'm a little new here, but I play the IL-2 series of games alot. Keep in mind though, this game only uses the older WW2 era planes that only have guns. Against AI's this almost always works because they aren't smart enough yet to see this coming.
The Cobra, I think, is best used in an agile one engined fighter against, say, a guy flying a two engined fighter like the Bf-110. Using it against anything else, one on one is just plain suicidal.
Although there was one experience:
I was in a Bf-109 with a Spit on my tail. I slowed down, dipped and dodged, dropped my flaps when i hit 250 kph, zoomed up, redlined my engines and climbed pretty quick.
Now what I didn't see was a guy flying a Zero coming in from high and behind. I went to the external camera and I saw what ended up being a mid-air between the Spit and the Zero.
SO other than that one experience, I've never had any other use for the maneuver, i usually can't dodge the bullets enough to save my own skin or my plane.