Regenerative doesn't have any effect from boosting, and no benefit to having attuned over just the lowest level. It's a flat bonus that works like a set bonus. The 3% in steadfast also works like a set bonus, but it has a resist component to it as well. So for that it would depend on whether you exemp a lot or not. You'd have to compare in pine's the different levels with or without boosts to find your sweet spot for the content you do most. If you never exemp, boosted top level would be optimal. Personally I just get attuned and forget it with that one