I have one on a mini. Definitely an improvement over factory, but the jury is still out out on HOW MUCH of an improvement. I just got the rifle back form the smith. See my post in the Gallery "Tang's CQB Mini". You can see the mini with the barrel installed, and I have also posted a couple of test targets. You can see for yourself what the barrel has done for me thus far.
I expect once the bedding settles in and I can try a few more loads I can get her to do MOA.