Ah - if your powder is already sweetened, then you should be ok. The texture is really dependant on how thick or thin you make them. If you spoon them out slightly thicker, then you should be fine. They hold together really well once you add the topping.

Since the topping is the same recipe as the bars minus the protein powder, I'd microwave the butter and pb together, add the other ingredients, and then give it a taste. If it needs a little bit of sweetner, you can add there. I have found it doesn't, but everyone's taste is different.