I don't have any safety evidence/stats to give you.
But, my oldest (9) rides in the 3rd row. He's currently in the center (which we know is the safest seating position) in a PWSG. I like him there because...as many of us do, we put the "least protected" in the "most protected" position. I don't consider him unsafe in a HBB, but I do feel like he's safer in the center of the 3rd row vs. outboard 3rd row. He also has more legroom room in the center and there's nothing directly in front of his face.
That said, when he outgrows the back on the PWSG, he's going to have to move outboard in a backless because the shoulder belt outboard fits better then the center shoulder belt AND I like the outboard headrests better. He'll still be pretty darn safe back there because 1) his seatbelt will fit properly, 2) his LBB fits him well (and fits in my vehicle well), 3) he has adequate head support, 4) the 3rd row has curtains, and 5) for everyday travel, all of our stuff in the cargo rests below the seatback level in the cargo "well" (makes me feel a little better about cargo loading into the 3rd row seatback).
So, I'd say...if your child fits well in the booster, and there's a good head support and seatbelt fit, the booster fits well in your vehicle, and your chid rides properly the whole ride, I, personally, wouldn't spend too much time stressing over it.
