For me, running mechanical filtration helps extend the life and effectiveness of GFO, and can stretch your maintenance intervals a little bit. It keeps the foam pads in the upflow reactor stay cleaner for a longer time. So the water moves better longer, the granules vibrate as they should, flow doesn't slow down, there is less organic decomposing going on and less caking. What's not to like, huh?
I have proven all this to myself many times over the years when I decided to run with mechanical filtration removed for a time, but still kept the GFO reactor on line & running. In these cases flow would slow down early, I could see channeling happening in the media which means some water is going in the tank only through an exhausted vein of media whole other parts are not being used and are stagnating.
Also the proper pump size aka flow is critical, and making sure your reactor is level helps a lot.