I'm lost as the why they made the buff in the first place. I don't think any one would have asked for something like this in the first place.
Her R has to be the finishing hit to receive the benefit is has, it doesn't do anything to Lux's playstyle other than make an incentive to delay R's and snipe more often. Nothing added to when she's behind... where R would do noticeably less damage.
A buff better suited, I think at least, would be to move some damage off her Ult and increase her passive's base and ratio damage and ease it with a change from the .75 ratio to something else.
That way, her early game is more suited to be riskier - while her Ult is only good, if not stronger while ahead, if she blasts it in a rotation. Of course, I think this kind of buff would have been only good if Thunderlord's didn't distort her trading patterns and overall combo's damage.