Breastfed babies do not gain less weight then formula fed babies--that is incorrect information. If a CF child is having stool issues due to pancreatic insuffiency, it won't matter the source of food or even how frequently they are eating. They aren't gaining as well because of malabsorption of fat and protein. Period.
However, the stool thing you gotta watch and if it continues I would ask for enzymes to try for a few weeks to see if her stools and weight change. Not all Pan Insuf. shows itself at first. In fact it can be harder to detect in breastfed babies because of the natural lipase (enzyme) found in it and the looser/watery consistency of breastfed poops. However, smelly, greasy, frothy, greenish, or bloody stools are signs of malabsorption in any baby, especially breastfed because they shouldn't smell at all (normal ones smell like yogurt with little smell). Some moms I've talked to (again breastfed CF babies) didn't get diagnosed until much later when malabsorption showed up as they got older or started eating solids.
There isn't necessarily anything wrong with the 10% either...it is the growth curve you want to look at. Is she gaining consistenty over time at a good rate (typically 4-6 ounces a week or 1- 1 1/2lbs a month for most babies)? This is what is hard to deal with sometimes with CF docs. Even on enzymes, some babies simply hang out between the lower percentiles (formula or breast milk) until later in life. It's not a failure and it doesn't always indicate a problem. Not gaining weight or loosing weight is all together different.
You can give her a probiotic (like Culterelle or Kid Dophilous) once a day to help with absorption. If she needs enzymes this will just help even more. You can mix a little of the powder (1/2 cap of Culterlled) in apple sauce and dip a clean finger in it, letting her suck it off until it is gone. This will be how you give the enzymes if you end up needing them until she can eat off a spoon. Perhaps watch the stools for another week and get a weight check. If she keeps having what you described get the enzymes and then start the probiotic. If it seems to have been a fluke, then add the probiotics just cause its a great thing to do.
Keep us posted and you can always email me or PM if you need to.
Warmly,
Jody