It seems a little silly to me picturing an X-wing opening and closing its S-foils constantly mid-battle.
As a rule, the OP is fine--although personally I wouldn't use it much, since the ability to move just a little faster on the table isn't typically worth the inability to attack. You might make the speed bonus apply to straight or bank maneuvers, rather than just straight ones, to help make it a little more powerful.
As a piece of theme, I'm with those who don't see a need for it. Even at the maximum distance from an opposing ship (that is, corner to corner) you're never more than a moment from being attacked, which means it doesn't make sense to me to engage a flight mode that prevents you from firing.