I did some searches and didn't find any threads that indicated this topic was covered yet. Sorry if it's old news.
A few weeks ago, I noticed that the starbucks app let me order ahead for a power lunch and then apply my stars reward to my order. I picked up my power lunch and the app said I only was charged a little more than 4 dollars. I have asked the cashiers several times at several locations to apply the power lunch deal first and then apply my reward and they do one of two things:

a) deny that it is possible and offer me the choice of power lunch or star reward, not both.
b) try it and discover that it works. It first takes money off each item for the power lunch, then subtracts more money for redeeming the star reward. I think it usually takes the most expensive item in the power lunch at it's discounted price as a free item and that usually takes an additional $3.xx or so off the total. The register usually flashes a combined discount of -$8.xx which is more than the highest free food item I've been able to buy alone: a $6.95 salad.

I could be wrong, but I feel like this is a hot deal because you are walking out the door with all of the power lunch items for only about 4 bucks. When I use a star reward, I try to get the highest possible discount and so far, this appears to be the way to do that. Believe me, if I could use a star reward on something like a box of pods, I'd do it.

Power Lunch = sandwich or bistro box (maybe salad?) + fruit or fruit bar + chips + ethos water