A PC with Improved Full Throttle has successfully made his Hard Piloting check, increasing the Speed of his vehicle by 2. Would it be reasonable to assume the vehicle might suffer 2 SS if the pilot leaves the piloting to someone else who doesn't have IFT before the talent's benefit ends?

Could go either way.

The better pilot has improved the speed.

The lesser pilot cant maintain it....

But then they would need a maneouvre to swap positions...

Id judge it based on the ships current SS levels, and what your intent it.

I can imagine my players would gripe constantly about it... but I dont see an issue in your suggestion. But dont know if its RAW or RAI

It probably looses that extra speed if the expert pilot is no longer at the controls. Just think of a 2 seater race car with a championship winning driver at the wheel, then swap them for a driver who is always in the middle of the pack. They are both professional Drivers, but 1 gets a lot more out of the same equipment.

At best the new 'lesser' pilot could get 1 turn at 1 boosted speed before the ship is back to normal speed, depending on if they attempt to keep the speed high (and ultimately fail, because they just can't, perhaps have them suffer personal strain from the effort? Add a black die to their rolls that turn?) or if they don't risk it and immediately slow down to a speed they can handle.

Otherwise yea, what Richard said.

At best the new 'lesser' pilot could get 1 turn at 1 boosted speed before the ship is back to normal speed, depending on if they attempt to keep the speed high (and ultimately fail, because they just can't, perhaps have them suffer personal strain from the effort? Add a black die to their rolls that turn?) or if they don't risk it and immediately slow down to a speed they can handle.

Otherwise yea, what Richard said.

I know what I would be using Despair for