400+ grams of protein is probably just asking for trouble. When people get tunnel vision on protein they usually end up losing out on a lot of other nutrients and i've seen on several occasions wher epeople develope food allergies and stomach issues from eating like this.
If you are close to 20% BF and can't gain lean muscle on 250 grams of protein, the odds are there is another place in your training/diet that can be fixed and the end result is a healthier and stronger person than you could of been on a 500 gram protein diet.