What, exactly, would the increased uranium level do?

• It doesn't seem to me that it would speed up the development of an atomic bomb much because you have to have the idea in the first place; and in our timeline, the atomic bomb followed the idea very quickly (what was it, 6 years?); the lower concentration no doubt slowed things by a few months or perhaps less than 5 years, but the histories I read didn't point to concentrating as a bottleneck but more conceptual issues (how much do you need? how do the explosive lenses work? etc.)

  Nor do I see how it might speed up the general development of physics and study of radioactivity; if Marie Curie was willing to go through tons of pitchblende to get a minute bit of radium, then uranium clearly was nowhere on her radar. Going from 0.6 to 3% won't suddenly make a Curie study uranium ore instead.

  The one such path would be discovering a natural uranium reactor, but how big a window is there where scientists could discover a reactor and speed up development of nuclear physics? I mean, if a scientist in the 1700s had discovered a uranium reactor, would he be able to do anything about it? Or would it just remain a curiosity, something like the Greeks and magnets?

• Nuclear proliferation is not constrained by the ability to refine ore, but more by politics; South Africa and South Korea and Libya and Iraq didn't abandon their nukes or programs because it was costing them 6x as much to refine uranium.
• Nukes wouldn't become much more effective; nukes are so colossally expensive that their yields are set according to function and accuracy of targeting. (The poorer your targeting, like Russia, the bigger your yields will be to compensate.)