I have both a tune and a sprint booster and IMO there is a world of difference between just a tune, and a tune with a selectable SB. I turned my SB off the other day (it is 99% of the time in the green mid-tier setting) and I was amazed how used to it I had gotten and how much of a difference it makes in throttle response. Driving the 6 speed is easier because of it...

The tune is huge, and I would do first, but for anyone who drives spirited, especially in a manual, the SB is a very big improvement over stock that should not be forgotten when trying to wring out all the performance you can from the XF....