Speed Boost causes the Dornier’s attack to be resolved immediately. For example if the result of the attack was a damaged counter the defending plane would be destroyed.
As an example an Me 109 scores one hit on a UK Spitfire, which would be a face-down disrupted counter. Then a Dornier then uses it’s speed boost and scores a second hit, which would normally give the Spitfire a face-down destroyed counter, but because it’s immediately resolved the destroyed counter is turned face up and the spitfire is removed as a casualty; it won’t get to fire if that player went second.
Even if the Dornier only manages to disrupt an aircraft with speed boost, ground units can still target the aircraft against it’s now reduced defence (due to disruption) and it’s already taken one hit this turn, so only needs one more.
For all intents and purposes Ignores means you “treat as if it wasn’t there.” In the case of heroes this means that a disrupted counter has absolutely no effect on them - you might as well remove any face-up ones. Defencive-Fire has little effect on them* since it is disruption that causes them to stop moving. You don’t have to get 6 successes at once to take them out, as Heroes don’t ignore hits (ie face-down disruptions).
Unless the unit has Flamethrower, Covering Fire, etc, as these Special Abilities add another effect to the defencive-fire which has nothing to do with disruption.
