5 CFM should be more than enough. Although I no longer own a BGE I have used my Stoker with a 4 CFM fan to control temps on a large for over 2 years and had no problems. The key to holding stable temps in the BGE is learning what top damper adjustments work best for you. At times I had wild temp swings and found that there were three things that contributed to them,
1) The fire is directly below the temp probe, when the fan kicks on it blows the air up towards the temperature probe and the temp spikes. Try to position the probe toward the sides of the egg.
2) A good dome seal changes things dramatically, give the seal the dollar bill test, if it's snug all of the way around it's good, if not adjust the bands.
3) Spend time fiddling with the top damper openings. For > 350 the daisy wheel should be all of the way open and maybe even the other part, I forget what it's called, should be slid to the side a little.
I now use my 4 CFM to control a Stumps Clone with a 24" wide x 24" deep x 32" high cooking area and have no problems getting it past 350 Degrees. I also have a 10 CFM fan that I purchased for the clone and I find it to be massive overkill. It sits in a box in the garage.