 Yes, gapless microlenses have helped improve the sensitivity of photodiodes, but the big benefit of BSI is the removal of the transistors from the surface area. That way you maximize the area and volume of the light sensitive region of the photodiode. It provides a bit of a boost to full-well capacity in most cases. But I've heard there's a trade-off in terms of noise performance. What BSI sensor designs tend to gain in light sensitivity and improved photon (shot) noise performance, they end up losing a bit of read noise performance because the transistors are embedded deeper in the silicon where more dark current is generated. And yes, the potential of stacked sensor designs is perhaps the most significant advantage to be had with BSI architectures. 