Are there any differences between the cycles Environment texture node and the Image texture node besides the available projections?
Looking at the definitions for the Image Texture node and for the Environment Texture node, the only difference I note is the available projections. As you noted, Environment Texture offers Equirectangular and Mirror Ball, whereas Image Texture offers Flat and Box.
Some things both have in common are that both will unpremultiply the alpha data, and both will convert sRGB to scene-linear color space. Other than that (and the complications of the Box projection), the files are basically identical.