I have a 10.5" pistol for now. If my SBR stamp ever shows up, it'll finally be a rifle. I only shoot mine with a suppressor, whether it's supersonic or sub-sonic ammo.
I have an EOtech XPS 2-2 on mine and am getting about 2" groups at 100 yards with supersonic ammo. 110gr Vmax running right around 2300fps. The aiming dot is 1 moa to begin with so I'm pretty happy with 2" groups at that distance. Once I get my stamp, I'll put a scope on it to see what it'll do with some magnification.
I don't shoot cast bullets, my 220gr subsonic loads are around the 1050fps and the pistol guys say that you can start to get lead fouling at around the 8-900fps mark. YMMV