BUT - you don't need to have the whole thing rendered at that sort of detail.
In LO/FC you fly over an (admittedly large) island of detail on a larger featureless world.
In the same way that the detailed LO/FC map sits in a larger barren wilderness, for the FPS pick a number of areas of managable size (islands of high detail) say 10 - 15 fairly large ones, some rural, some urban - create detailed maps for them that are separated by the correct amount of distance and have gross structural simillarities to allow them to be correlated with areas on the LO/FC map.
( I guess you've played Far Cry - Same idea. Islands of detail in a much larger map that's covered in featureless water. )
In the countryside between you could have either:
A map exactly like LO/FC's with the same amount of detail as it has now - ie blank outside the Caucuses-Crimea, then FC/LO countryside & most cities (which is all there & damagable - if not very detailed by FPS standards) then several strategic towns, airbases & maybe open rural / valleys & their surounds all at FPS detail.
or
That map plus - When you create a map in the Far Cry sandbox (the only editor outside of LO/FC I've used so that's what I'll talk about) you define a set of rules to terrain that landscape between this hight and this hight, when the angle of the ground is between this & that there will be this texture on the ground, this plant at this density per sq meter, this second plant at this density etc. You could simply have a standard set of these rules for each season (smart enough to not plant over towns, roads & rivers) generate the textures / vegetation on the fly "Bubble wise" as you moved between those detailed areas. they wouldn't need to be particularly dense or detailed & if they're just non collidable sprites like the trees in LO/FC or the grass/bushes in Far Cry, it doesn't matter if people are able to customise the render distance / density as we can for trees in LO/FC. + the visible range is usually a lot lower in a FPS so they wouldn't have to generate more than a few miles around themselves (& make planes a special case for ground units visability so they don't get snuck up on.)
None of it would interfere with the SIM Frames per second as the flight sim wouldn't see any of it.
For the ground game you'd only end up with a low detail map of LO/FC size, but would have 15 maps or so of First person shooter quality spread over however many thousands of sq km there are in LO/FC (& you could travel over any of it!).
Ground players could chose to concentrate their activities around the more detailed islands, or cross country into the larger but less detailed LO/FC enviroment.