Well, a murmur that they're born with is usually less serious than a murmur that they develop when they're older so I wouldn't worry too much. I was born with a heart murmur and I'm just fine.
One of my Greyhounds has a heart murmur and it doesn't affect her in any way, and I had a cat with a murmur who died of something completely unrelated. I'm not a vet, but I'm not sure that I agree that Baggins is smaller because of a heart murmur, or that her energy level would be less because of it. I've never come across anything in my research to suggest that that would be the case. Again, I'm not a vet and I don't know the severity of the murmur so I could be very wrong, it's just my opinion. I would be more inclined to think that Baggins is just the runt of the litter and therefore smaller and less active.
As for food options, I would give a really good quality canned food for now. I find kittens tend to eat more canned and less dry so canned might help her growth. I prefer Wellness, but my cats don't like it. :roll: Merrick is another good one and cats seem to love it. I would stay away from anything you can buy at the grocery store or at the vet, they're full of not so good ingredients.