Hi Kevin,
A field flattener should work fine for photometry. There is a chance it might improve your photometry if it reduces any distortions in stars towards the edge of your field of view. Though the effect is probably going to be negligible.
If it is the one on the FLO website, then it also acts as a x0.8 focal reducer, so your focal length goes from 540mm (f/6) to 432mm (f/4.8). It would be a good idea to check the sky pixel size of this setup, though in all probability this will be about whether or not you bin the pixels.
Cheers,
Andy
