Having no experience with aluminum rockets, I don't really know how hard it is to get an exact fit. I would be willing to bet, though, that the current tolerance is better than most 6" rockets out there. I am quite happy with the fix, and if you want to be able to tell me "I told you so", then perhaps I could suggest a dozen or so things that are now more likely to cause a problem than this.
I'm actually more worried about the fit being too good, and affecting the apogee separation of the booster. The coupler is metal and is connected to the motor tube, which is metal. Although there are several things I can do with this design to minimize the actual metal-to-metal contact, I suspect the coupler will warm up at some point. This issue will get more consideration over the next few weeks.
Jim