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