I have three of these brakes and an assortment of their suppressor mounting pieces.
I was told when I was emailing Jon that the difference was extremely minimal. I still waited for the 6.5 to come off back order so I had the three sizes I wanted. Having installed both sizes on both my 6.5 gas gun and my bolt action, I can say that at least to my shoulder, I couldn't feel a difference really.
As far as the direct thread suppressor, I only have a couple hundred rounds direct threading a Sandman Ti with the system but no draw backs so far other than a tiny amount of added length.
I can say that I was really impressed with the attention to detail and machining quality on all the parts I received, and so far have been absolutely thrilled with the performance and flexibility of the brake system.