Just my 2 cents...
The style of the jacket is meant to be a bit blousy. Like someone said, look at how it looks on Steve McQueen in The Thomas Crown Affair or other photos with him wearing it. The Slim version is only meant to be slimmer, not skin tight like a modern designed leather jacket. This jacket is kind of a traditional golf jacket, which is meant to allow some sporting movement.
I have a 37.5" chest and ordered a small (size 38) slimfit MIE and while it almost fit fine, it was too snug in the shoulders, even with just a thin shirt underneath. The length was okay, as the waistband fit over my beltline (I would not mind another .5") and the sleeves were just about right, but I wouldn't have minded another .25"-.5". I am 6'3" and 165lbs. I have long arms I guess, so I wasn't too surprised. Anyway, the body fit alright, except for the shoulders area. If i zipped it all the way up, you could tell that it was pulling a little. I must say that I do not have what anyone would call broad shoulders! Even though some parts of the jacket kind of fit, I felt overall that it was a bit small for the cut that the jacket is meant to be. I felt like I was a bit restricted in arm movement, and that is not what this thing is about. I sold it here on the marketplace and I'm now awaiting a size 40 slimfit that should be here any day.
I have a feeling that this will be just about perfect. More shoulder room obviously, and a little more sleeve and body length. Yes, around the chest it will be a little more roomy, but I don't think it should be so much more roomy that it looks wrong considering the style of the jacket. It will probably still look more fitted than the classic G9. I generally wear Medium in shirts and jackets anyway and the 38 is technically a small.
It is strange though since I feel like I'm the first one who has written about not having enough shoulder room and I would have never expected to have this problem.