aehm...
I don't want to sound annoying, and it's up to you obviously (since you want/are doing it), BUT....
8x 2048x2048 means more than an overall 8000x4000 texture size
even if you face directly the model, I doubt you'll see less than 50% of the semisphere facing you, and it's a lossy approximation. I mean, like if you are zooming the model so you can see only 50% of it.
Which mean not less than 4000x2000 texture's pixels showed at anytime on screen....and I doubt there will ever be many people playing at higher res than 1600x1200.
Now, I admit I have no idea if you still get visual improovements when you have more texture's pixels than your screen pixels, probably yes, but to me it sound like a waste, expecially considering that many here got excellent high quality background spheres with far, far lower texture sizes.
Anyway...I wasn't talking about tileing, but about uvmaps borders not matching the texture's borders (althought it shouldn't happen).
My suggestion is to use 6 planar maps instead of 8 (top,bottom,right,left,front,back) generated with baking out of the 10800 map, and unwrap the model in Lithunwrap (althought there should be similar tools for lightwave or max). Here you will be able to reduce or increase the size of the uvmaps over the textures(if the problem is the texture's borders).
nfact, this problem could be caused by the anti-aliasing being activated while cutting in photoshop (actually this is an error that many do). edit: as you said, there aren't anomalies when putting together the textures so it shouldn't be this
I *may* try to help you but I dare my pc would explode trying to open that...thing....