I normally use a 10 mph delta when P&Ging. 15 is nice when traffic allows. I rarely do 20 unless its a totally abandoned road and I'm really in no hurry at all.
I normally accelerate around 70-85 LOD.
The benefit of P&G is better mileage over a long distance.
Your FE will plateau. However, if you go back to driving normally it'll go down. P&G is just more efficient.