Couatl is not a download manager, you are maybe confusing it with the Addon Manager, which is called a "manager", but it doesn't handle downloads.
About the Addon Manager, as with any other .DLL module (you probably have many of them installed), it's not possible to shut it down and restart it without restarting FSX too. Of course, the Addon Manager doesn't do *anything* if you are not flying over FSDT airport, other than checking with a VERY low priority thread which doesn't consume any CPU time that one could be able to measure, if you are entering in an area covered by an FSDT scenery, so it will start to do its things.
Couatl.EXE, instead, doesn't take any valuable CPU resources, because it's an external .EXE (that was the main reason to do it this way) and, being an external .EXE, it can't slow down FSX and it doesn't waste any CPU cycles that FSX might use, because separate .EXE files are automatically scheduled by the OS to take spare CPU cycles from different cores, and since FSX never uses all available cores, on a multi-core system, it's 100% sure that there ARE spare CPU cores that Couatl can be assigned to, so it won't use any of the "valuable CPU cycles needed by FSX". And of course, the same reasoning is valid for Couatl too, that it doesn't do anything if you are not flying on an airport (in case of GSX, any airport, since it works everywhere) AND you are landed.
So, Couatl automatically does what you are are asking: it doesn't do anything (and even if it did, it would use spare CPU cycles as explained before), unless you are landed on an airport, and of course even after you landed, it doesn't start doing complex calculations, unless you call an actual GSX service. But again, even those calculations can't slow down FSX, since they run on a separate thread which is scheduled by the OS to run on a spare core.
The little (if any) fps impact that GSX has is NOT caused by "valuable CPU cycles" being wasted by Couatl but, instead, by the actual graphic models of the GSX vehicles themselves, which might compare to adding 3-4 more AI airplanes.
You can verify this quite easily: go to any default airport (not FSDT, because Couatl is needed there to generate the scenery) and don't call GSX, just sit there and look at the frame rate. Then, open the Task Manager and kill the Couatl.exe process: you wil see the fps hasn't changed a bit.
