There's no performance hit whatsoever in regular flying.
First, because no GSX objects are drawn until you call GSX AND, because the extremely low impact processing that GSX does in the background, can't impact FSX fps, because it's running entirely externally to it, since the whole GSX code runs in the Couatl.exe interpreter that, being an external .EXE, can't affect FSX fps, since it will be scheduled by Windows to use spare CPU cycles, that on a multi-core CPU are plenty, since FSX can't use more than 20-30% of a multi-core PC.
When parked and calling GSX vehicles, their fps impact might be comparable to a couple of AI airplanes more. Considering that with some additional AI traffic package, you can have more than hundred AI at a large airport, adding a couple more shouldn't be noticeable.
And of course, since GSX is available in Trial version, you can check yourself on your system how it works before purchasing it.