Certainly completing the Fellsman is a good steppng stone to building up the confidence in your own mental and physical stamina that is necessary to do the BGR. Others may have a different view but I don't think you can compare the routes as one is boggy and rough and the other is relatively rocky, firm going with mostly good paths and, of course well over double the ascent. There is no simplistic answer as it would be misleading to say one is twice as hard or 150% as hard.