First, the airport loading distance was never always 10 NM, but it changed depending on the airport. The largest ones, like KMEM or KDFW or KLAX, had it at 16.0, for example.
The software is already doing it at the object level so, all object ranges are automatically doubled when running under P3D V4, and the anti pop-up setting default to its maximum, so you already seeing a big difference and far less popups.
Thanks to our custom management in our software, this can be done very easily, without having to update all the sceneries.
However, we are a bit cautious with arbitrary enlarge the overall airport loading range, because in some cases it was chosen for a precise reason.
Case in point: KJFK, which is so close to KLGA and the dense area of N.Y.C. We set a loading range carefully tuned to prevent the loading of KJFK when you were at KLGA, and in FSX this was absolutely necessary to prevent OOMs but, while in P3D V4 OOMs might not be an issue, frame rate will still be a problem.
or at least make a distance slider for each airport so depending on the # of airports in a given locale (nyc area vs. phnl), this can be optimized?
No, that's just too confusing and, history has proven that, whenever there's a slider, users will always set it to the (usually wrong) highest value. We'll probably increase the preset loading range for the less critical airports.