Chris,
I was looking at the delay that occurs (on my machine anyway) when the next level of texture loads and wondering if you load all the tiles at that particular resolution that are in the field of view. (I am assuming that you do but correct me if I am wrong). If that is the case then I thought there may be some speed improvement by not increasing the tile texture level for tiles which are viewed at a steep angle. For example if you are looking directly toward the centre of planet as you zoom down then the tile textures need to load as they do now. But if I am at level 5 and I look toward the horizon then tiles at the horizon probably look no different whether they are displayed at level 5 or level 2. Because they are viewed at such an acute angle there is little opportunity to see any detail. My thoughts are that the tiles within a certain radius of the observer need to be at the appropriate level and that at increasing radii the levels should drop away perhaps to level 0 at the horizon. This could radically reduce the number of tiles that need loading especially when in close without affecting the image quality.
ron