I've assumed those weaker powers are speed bumps for better powers down the tree, so swapping them is something I'd be really careful with.
I think a straight respec would be good enough a reward for most missions.
I guess it may be overpowered, that's what Klutz mentioned the other day. My consideration for this was to make some combinations available that previously were not. I also thought that the variety in combat can get a little dull, and that it could be more interesting when you get some nice upgrade combinations.
The upshot of Vision Quest would be that you open up possibilities that were not feasible before. It seems like the relative weakness or strength of a power has little or nothing to do with tier. For instance if you do Vision Quest to get Forge Works, it is really hard to say it is a stronger ability than void enhancement. For most of my Ranchers it is not even as strong as iron concentration.