What custom ground method ? There are many, and we used several variation of it with our products along the years.
Most of the developers out there (including ourselves for several sceneries) haven't invented any new method for FSX, they rely on the fact that FSX, after SP1, is able to conform old style FS8 polygons to the curvature of the earth which FSX supports.
This is convenient, because it allows to use the same FS9 file and methods for both FSX and FS9, but with the side effect the materials can't be of the native FSX type (no shaders, no specular effect, no advanced blending modes to choose from) AND it has a strong impact on performances. Just the yellow ground lines, for example, at Zurich, impact fps more than all the buildings. Not that Zurich has bad fps but, by freeing pc resources used to draw ground, we can increase complexity in other areas, which is why KDFW (and especially KLAX) are significantly more polygonally complex than Zurich, without being any slower.
Starting from KDFW, we use an entirely different method, fully FSX with no FS8 polygons whatsoever, which allow to use FSX native materials with shaders and it's way easier on fps, which is why KDFW has good fps despite of its huge size. This method can't be used without the support of the Addon Manager, which works in the background to eliminate what would be horrible flickering and problems with airplane shadows. Another advantage of this method is that, by being able to use FSX materials and the advanced blending it provides, we can also save A LOT of polygons, by not having to use any polygons at all when specifying which part of the ground gets which type of detail texture, because a shaders does it for us.
Note that, it could be possible to not have to rely on the additional Addon Manager help, when doing a very small airfield, were the effect of the earth curvature on a small distance wouldn't be enough to have visual problems BUT, on big airports like those we usually do, it couldn't be done using official SDK methods only.
