Yeah, I had it done on two '04 Echo Control frames and would happily claim they were the strongest brake mounts I've ever run on a frame There was around 1mm of solid material all the way around the insert, which I wasn't too sure about at first, but after checking it all out and thinking long and hard I went ahead with it and certainly wouldn't look back You could always knock up something just a touch larger than M6 to give yourself a little more surrounding material if you'd prefer/need to The reason you won't find much smaller standard inner/outer pitch style inserts is due to the fact this'll lead to regions of very little material within the insert, making the stress distributions within irregular I would imagine (See Diagram) You need to keep a tihck enough wall for this not to be much of an issue really As you can see, the thinner the wall of the insert the closer the two threads are together, and the more likely this thin region is to become too small/impossible A Helicoil style insert has the same pitch inside and out, so won't have this issue and is a coil sitting in it's own thread as opposed to a solid peice