LO,
Stealth is governed by the law of diminishing returns. Adding a bit of stealth is quite cheap - call it signature reduction. Even an 80% solution isn't too bad, but beyond that it gets really expensive and the broader the bandwidth you want to cover the worse it gets. Because of its size, JSF was always going to be a tough one which is why it suffers in the rear sector and down in the VLF region. B2 is a much better size and shape for this kind of thing.
As much of its stealth in is its shape, there's not that much that can be done to develop it further, so as it falls behind, it stays behind.
Given the choice, I'd go for the stand-off weapons next.