Here's another thing - what's the cost of the handling upgrades? $20,000, $27,000 and $35,000 right - for Smooth Handling / Sport Handling / Race Handling respectively?
Well that's the exact same price as the brake upgrades on top-end vehicles...
I'm calling bullsh*t on the handling doing anything to the planes themselves and they don't even brake differently when this upgrade is fitted anyway (if that's what it was supposed to do in reference to that handling upgrade being VMT_BRAKES). Seeing as the engine stuff hasn't proven to me that it's any faster over the stock version either, my guess here is that Rockstar were lazy and those upgrades aren't applying to the proper areas.
For example - I'm not sure how it works but if VMT_ENGINE and VMT_BRAKES is supposed to multiply or increase the braking bias or force and engine rpm of vehicles - that's all well and good - but aircraft have their specific "CFlyingHandlingData" with values pertaining to pitch, roll and yaw, as well as thrust - which probably aren't being multiplied properly if Rockstar cocked up somewhere.