I think the sizing at LB is different because they're made for Asian guys? I had to order a B-XL and an A-2XL, and I'm generally an M at everything else.
The Super Strength Velcro Binder is very sturdy and strong, and in my opinion does a much better job than the High Performance. Personally I have no problems with armpit fat with either one, but my arms aren't where the fat piles up for me so it might be different for others
I have to wear one on top of the other for decent results, but I'm a large D cup (or higher, I don't even know)
One thing you can do if the size is just a little large, is add some more velcro stick tape in the back, so you can make it tighter if you want. I did that and it works perfect