What is the difference in results for BBL for fat layered into the muscle vs injected into the superficial layer only?

Fat embolus is a very serious and known complication of fat transfer (which has been recently spotlighted in the media). While it is true, injection of donor fat into the infra-muscular plane (within the muscle) increases fat viability this does come with a significant increase in risk. There are large vessels deep to and within the muscle which can be inadvertently injected while grafting leading to a fat embolus. The consequences of such a complication are very serious and life threatening.
At the recent American Society of Plastic Surgeons Hot Topics (2016), data from AAAASF (ambulatory surgical certifying body) was presented, suggesting that BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund is convening a task force as well. The hope is that we will have more concrete guidelines for addressing these very serious complications.
That being said, the safest way to avoid fat emboli is to stay in the subcutaneous plane (at the expense of fat viability)and avoid the deeper muscular plane. If grafting of the deep plane is planned, surgeons should consider blunt cannulas, aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed.
With regards to your specific question, it is difficult to quantify the difference definitively but there is a decrease in graft viability/take when placed in the subcutaneous plane. The vascularity of the muscle and the subsequent graft viability are directly related. However, as I described above, so too are the risks.
In summary, the procedure can be safely performed, however, they should be performed by a board certified (ABPS) plastic surgeon with experience. These risks should be discussed as well as the strategies being employed to avoid them.

 While it is true that fat injections into the deep gluteal blood vessels is hazardous, this complication can be avoided if the surgeon is aware of the pertinent anatomy and exercises good surgical judgement as he or she injects fat into the muscle.  The results are better because augmenting muscle gives improved fat retention, and better contours.