So far there has been no information released about flight mechanics (and not much on movement in general). Earlier during development Larian did say they were trying to keep the combat mechanics as similar as possible between dragon slayer and dragon forms. Since the dragon slayer can jump and dodge/roll around during combat, that could mean it may be possible for the dragon to roll, etc.
As for speed, I would guess there may be the same walk/run toggle for both forms. I don't think fine, continuous control over speed is necessary, but it would be nice if the dragon speed up during a dive, and slowed when climbing.
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