There was a time when I too thought that altering the shoulders of a jacket was not for the faint of heart nor for those for whom a full bespoke tailor of tremendous skill was not available. I have since learned that this is pure nonsense. I've had the shoulders narrowed on two new Kiton jackets, one new Brioni jacket, and the jacket of one new Isaia suit. The tailors that did the work do make MTM suits and jackets here in Vancouver, but they are certainly not at the level of a Rafael, Shattuck, or Despos. The two tailors involved seemed to think that it wasn't that big a deal. The sleeves had to come off, of course, and to some, the idea of removing the sleeves of a high-end Neapolitan jacket like a Kiton brings tears to their eyes. But is it better to leave the shoulders oversize, producing the look of an ill-fitting garment just to retain all that original Neapolitan hand-stitching? I didn't think so, and have been very happy with every one of the shoulder alterations I have had done and noted above. Cost ran around $125 Cdn., or about $105 US--definitely well worth the price for a properly-fitting (and still top-shelf) jacket.
At the same time, on some of these, I had the waist nipped a bit too, producing a better-fitting garment; this might have added $30 or so to the total cost. The drape seemed unaffected (more of a factor with the Kiton and Isaia jackets). The overall result in all cases was a jacket that looked much more as though it had been made to measure than previously. I couldn't detect any problems that resulted from these alterations. Altering jacket length, however, is a whole other phenomenon, and that is probably something I wouldn't undertake.