The thing about powerfeeding is that whoever you ask, there are different definitions. Theres no real 100% accepted definition of powerfeeding, that is accepted by all reptile enthusiasts.
But I do have to agree, two med rats every three days is definately a contributing factor to her fasting. One small rat every 7 days is great, maybe even ONE med rat every 10 days. In my opinion 2 med rats every 3 days will lead to obesity and will cut her lifespan. As far as not going with a large one, thats good. Nobody is saying that feeding two prey items is bad, its the fact that you feed every 3 days and the size is med for both. You said she is 8 years old. She doesnt need to be fed every 3 days. Most people feed often if they are babies while they are growing. After about a year to two years you can switch to the 7 days method or the 10-14 days. I have a one year old myself that eats every 7 days one small rat. I don't think I'll switch her to med till breeding and I will NEVER give her two med. rats.
Oh, and when I say contributing factor, I mean that what Ive read and heard is that if they do overeat they will fast to sort of 'catch up'.