For omnidirectional passive antennas, there is only one way to increase the gain: flattening the radiation pattern. This means that when pushing the range limits you will lose signal every time you bank, climb or dive the plane. Useless IMHO.
I prefer to design omni antennas with the lowest possible gain, and increase the gain in the receiver end. This way you can fly much more acrobatic without video glitches (except if you point the radiation null directly to the receiver, what is quite difficult to achieve in normal flight).