I can tell you exactly. The gain is 0.
With respect to the score, the most important thing in ZONE is a perfect
lap. Doing perfect laps is a bit easier in lowers zones, right? Hence, as long
as the number of zones is low you have to run around the track as fast as
possible (using every speed pad) such that you can do a lot of perfect laps
until it becomes a bit more difficult. Further, pulling both airbrakes at
higher speeds will almost always offsets the craft for the worse. But there
is something to slow down the craft for a brief moment worth considering.
Once you get air-time the craft slows down considerably at very high
speeds (zones). In some situation this can be very helpful.