Okay, I have finally gotten one million hearts, and i will be giving it all ot palutena in 50 thousand increments.
So after giving her 1 million hearts... she is pretty close to the screen. I can only see from the waist up. I checked the prices of the weapons that I can buy and sell, and there is no notable difference. Fusing weapons doesn't add anything new, either.
What I did find, however, is that when I played a chapter, I found a bit of new dialogue between Pit and Palutena. I was playing chapter three, and Pit randomly began to talk about his goldfish. This may or may not be related to the heart offering or the intensity of the level(I was playing it at 8.5).
Also, the amount of hearts I could get in a level did not change. However, when I obtained weapons from chests, they seemed to be better than they usually were, such as more frequent weapons in the 300 value range with good ranged, melee, and attributes. That is about all I can see at the moment. Hope this cleared things up a bit.