I've never known a
Philippa, either. I think an American
Philippa would probably have to repeat her name to new people whom she meets, and probably hear a lot of, "I've never heard that name before", but it's not really too odd to be considered unusable. I really don't like the name, though. I tend not to like feminizations that sound just like the male name with an A tacked onto it, and
Philippa the way you want to pronounce it sounds just like that.
Philippa with the stress on the second syllable is better, but only marginally.