I have a good number of rifles in numerous calibers and chamberings. Many have match grade or custom barrels (Bartlein, Rock Creek, Hart, X-Caliber, Krieger, Broughton, etc.). Most are heavy contours, Rem Varmint and up. Some are heavy sporter weight barrels. With these barrels and proper load development, I've seen little to no benefit using a barrel tuner. I've tried several, including the Limb Savers and tunable muzzle brakes.
With that being said, I've had EXCELLENT results with the BOSS system on six of my sporter weight factory barreled Winchester M70 Classics, in various chamberings, even after load development. I've helped adjust many more BOSS equipped Winchesters and Brownings. The BOSS system may be ugly, but IT WORKS! Several of my BOSS equipped rifles shoot 5 round groups right at or less than 1/2 MOA. I have also successfully further reduced group sizes, after load development, in several rifles with factory barrels, using the Limb Saver dampeners.
Do they always help? Not always. A bad or shot-out barrel is a bad barrel, and external weights, vibration dampeners or a BOSS type system is going to do little in the way of improving accuracy, to an acceptable level, in a barrel suffering these issues.
On some rifles, the barrels just don't respond well to the tuner, no matter where you place it or how much you adjust it. Tuners or vibration dampeners do not always help but, I have seen plenty of times that they did.