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edit the flingy.dat file with Arsenal III changing the speed and acceleration for whatever buildings you want to change
That doesn't work unfortunately, building speed is actually hard coded, even though you can change those values. In mapping, if a building starts off in transit, it goes twice as fast as usual or something, in case you're interested.
The problem I see here is that I think the unit will still pickup targets while landed and not pickup targets when it's lifted-off, because of the orders it has (on the AI/orders tab). Also, the __3f_condjmp command in iscript will jump to a code block if the current unit is a lifted-off building if that helps you.