Two other advantages are: - The filled vias provide very efficient heat sinking, particularly in the area under the IC. I also saw a presentation recently by Amkor Anam, where they showed data that adding a 4 x 4 matrix of balls under the IC, tied to the die attach pad, gave a significant reduction in thermal resistance. Combine this with your approach to lower the thermal resistance. - Using vias as you suggest allows the repair of individual balls that may not have reflowed properly, by touching up with a soldering iron and solder wire from the bottom side. Let us know how you get on. Olly