I gained 26 lbs. with my pregnancy that went to 39+4 wks. I gave birth ******lly to a healthy, 7 lb. 10 oz. baby boy. It is my understanding that cf moms have a greater risk for premature labor more due to fetal nutrition than respiratory involvement. In order to gain the 26 lbs I had to eat ~5,000 calories a day. I was PI but had NEVER had issues keeping weight on. I had a great pregnancy in every way but also experienced rapid weight loss in the days after my son's birth.
I think its a good idea to plan for 25-30 lbs and chart it out on a calendar with weekly weight gain goals. My doctor recommended front loading because often the 3rd tri mommy doesn't have much room for food in her stomach so she fills quickly. It's hard to eat when you haven't got an appetite. I didn't gain a ton in the 1st tri but worked agressively in the 2nd tri and then the 3rd was able to lighten up because I had worked hard at eating (LOL I know!). I still gained on track the last tri and never did have issues eating normal servings probably because I am long torso-ed.