I think in pounds, so allow me to convert. You are considering adding roughly 1.3 pounds of 50/50 wheat LME to your barleywine wort. I do not think you have much to worry about for clarity, especially with the length of time a barleywine conditions. I also do not think you have to worry about that little amount of wheat affecting your flavor and aroma profiles.
That said, though, I hate jury-rigging recipes at the last moment just to reach some target volume and gravity. I would rather forgo the other two liters and work with the wort you got from the mash, adjusting the sugar and date syrup amounts accordingly.