OpenGL has a limit of 4096x4096.
If even 1 pixel of your 2048x2048 texture is showing, it will still be the same as processing the 2048x2048 texture.
The only time it pays to have multiple objects on the same UV texture, is when those objects are all in the same scene.Then it will only save you the chop when the GE loads and unloads new textures.
If your textures are not square the GE will blit your texture over a black bitmap buffer that is square to your biggest dimension. (if you have a 64x128 texture, it will make a 128x128 buffer, then blit your texture over the top half.)
Why are you doing this? The way you are mapping it to a plane makes me assume that you are doing it for a terrain mesh. If this is the case, you may want to just make a material with a bunch of small seamless terrain textures, and one large mix map texture to define what texture goes where.You will be able to retain a higher resolution on your seamless textures so things look better.
Please excuse my jumping in here. I have been playing with the Blender GE for the past couple of months.We are switching over to Crystalspace right now. If things dont work out in Crystalspace we may be forced to buy a license for Half Life 2 with hammer.