I think Medivacs need an OCCASIONAL speed burst thats pressured to be saved ONLY for escape situations, namely for evacuating your expensive thors from a sticky situation since their racial tier 3 ground equivalents are so much more mobile.
That being said, I would just suggest that ignite afterburners would take a lot more energy, or have a considerably long cooldown time. Or something.
Thank you and goodnight.
Also a sweet animation for when it was used would be nice. If the exhaust changed colors or something.