We are implementing HDR in an RGBM encoded LDR (RGBA8) render target in our engine. We can only support this on devices that support framebuffer fetch. The 5XX series have the GL_EXT_shader_framebuffer_fetch extension enumerated but it doesn't appear to work properly. The same shader code works fine on PowerVR, Mali, and iOS. Here are some screenshots and driver info.
no fetch (what it should look like): https://dl.dropboxusercontent.com/u/39900018/Screenshot_20161201-163636.png
Device Info: https://dl.dropboxusercontent.com/u/39900018/Screenshot_20161201-163256.png (I have also tested on a 510)