Generation of lower resolution textures
I would think rendering to a downsampled FBO would be much faster than generating mipmaps.  glGenerateMipmap will generate the complete mipmap pyramid, which seems wasteful:

https://www.khronos.org/opengles/sdk/docs/man/xhtml/glGenerateMipmap.xml

Besides, only OpenGL ES 3.0 lets you manually select mip level when texturing:

https://www.khronos.org/opengles/sdk/docs/man3/html/textureLod.xhtml

Artistically, this might not give you the look you want.  In my experience, simply downsampling gives a grainy look.  I would recommend also doing a pair of one-dimensional blurs, as described in this article from GPU Gems:

http://developer.download.nvidia.com/books/HTML/gpugems/gpugems_ch21.html