KJ, this is a fine way of looking at things, but there's a line between items looking kitschy and oversized (good) and sloppy (not good). If the shoulders fit, but the sleeves are long, fine; if the sleeves fit but the shoulders drop a bit, that's fine too - it's when both are off that I think it verges on sloppiness. The whole thing looks fairly droopy in that picture. I shouldn't say that it absolutely doesn't work, but in the low-quality bathroom picture you posted, it certainly doesn't appear to. I hope it works out, but when you're talking about tailoring one or more aspect of every purchase you make, it seems to me that you're just buying things in the wrong size.
I think it looks like a fun piece, though, especially with some loose white trousers and a ridiculous 70's shirt. FWIW, I don't think you get any "hate" at all. Snark, sure, but SF has a way of producing snark when advice is seemingly ignored, though.
Feel absolutely free to post more pictures and prove me completely wrong, though - maybe a full body shot when you're not hunching over with your hands in your pockets?
I agree with you on pretty much all points, here, except the one on sizing down. My issue is that I'm a straight 50 in the chest and shoulders, (maybe slightly smaller than average in the shoulders but not enough to throw anything off), but with the exception of most knits and some shirts I usually end up having to chop off an inch or two in the sleeves. I think if the sleeves are off on something, it will usually exacerbate anything else wrong with the fit, and makes the whole thing come off as sloppy.
I guess what I'm saying is that I probably need to spend the $15 at the tailor before posting pics, or else elucidate better than I did above that it needs to be adjusted before I really wear it. Here's the jacket with the cuffs flipped up inside the sleeves (excuse the bedhead) and in proper daylight:
The sleeves are now a little short and look weird without the cuffs but you get the idea. The way you described is pretty much exactly how I would style it, too. Also, aviators.