It's certainly easy enough to just get rid of specular reflection entirely if we want. I can rip out all the specularity code entirely if you want, or I can just disable specular rendering for the moment. I'm inclined to do the latter, but let me know what you think.
In terms of the "overwriting textures" issue, part of the reason I did things the way I did was so that users could actually switch which textures they were loading without having to deal with actually deleting or re-naming files. Unless I don't understand the way that the pk3 files work, if you have the high-res 1geoscape.pk3 file on your system, there's no way to tell the game to use the low-res 0geoscape.pk3 without actually removing the 1geoscape.pk3 file from the search path. It's still entirely feasible to have different pk3 files for the different texture resolutions the way I've done it, but if you have several of them on your system, my method uses the correct one based on the in-game options settings, rather than automatically using the highest resolution one that exists. This way, you could (for example) include 0geoscape.pk3 *and* 1geoscape.pk3 in the "standard" download, and then have 2geoscape.pk3 as an "optional" addon with the very-high-res textures. You could even include all three .pk3 files in the "standard" download without having to fear that you would be forcing people to use the high-res textures. It would make a difference in terms of hard-drive space and download bandwidth, but that would be the only difference for players with low-end systems.
I'll take a look at the issue with the sun effects being in the wrong place; I didn't try running the game at different resolutions, which I'm pretty sure is causing that problem. I don't think it should be too hard to fix.
I'll also go ahead and make the post-processing switchable at runtime. In terms of the "default" behavior, I'm not sure how easy it would be to enable/disable things by default based on auto-detection; is there an easy way of doing that? It will disable things if the system can't handle the new features at all (ie. the graphics card doesn't support the opengl extensions we need), but I'm not quite sure what to do beyond that. Though once I make it possible to turn post-processing on or off on-the-fly, it may not matter that much.
I'll let you know when I've got a new patch; in the mean time, let me know if anyone runs into any other issues.