I think so much depends on your baby, which you can't predict. A lot of people say their newborns sleep all of the time and it's totally fine, but my baby does NOT sleep all of the time and at 5.5 weeks, there hasn't been a single day since she was born that it would have been anything but a nightmare to fly with her, or spend any amount of time walking around WDW. She usually lasts about 30 minutes on our outings.
However, I don't think you'd need to worry about pumping, since you'll have the baby there with you, and if BFing is working well for you (another thing you can't predict) you might have better luck with soothing your baby than I do. To me this is one of those decisions where you have to ask yourself how badly you need to make the trip, and if it's worth the possibility that it will not go well.