It seems good. When I tried to do some exotic tiling terrain system, I used tricks of this way. And in the end I faced the same speed problem as you (as well as problem with the alpha not being retained, knowing that anyway it would maybe not a good idea to rely on the framebuffer having 32 bits of color depth). The main problem in the end for me was the mipmapping: I needed to have mipmap on, but each time I copied from the framebuffer to the texture this damd Blitz recreated the lower mipmap levels, very slowly and not in a way I could hide it (it tends to lock up the rendering). If we could set all the mipmappings levels ourselves, I'm pretty sure the overall system would be fast enough.
Anyway, you maybe don't need to have mipmapping on at all for your generated skybox (as its resolution is smaller than the screen resolution, and the maximum angle from which faces are seen shouldn't be too high). So if it's currently on, try to deactivate it and see if it helps. If it's not, well, sorry for this blabla :)
Anyway, you maybe don't need to have mipmapping on at all for your generated skybox (as its resolution is smaller than the screen resolution, and the maximum angle from which faces are seen shouldn't be too high). So if it's currently on, try to deactivate it and see if it helps. If it's not, well, sorry for this blabla :)