I wear a 40/50 suit and 15.75 shirt. Bear in mind that the SuitSupply sizing is pretty inconsistent within their different cuts. It might just be that I have tried on different Napolis than you have. That's another issue I would fix, all suits of a certain cut should fit the same. It can be pretty annoying when for example a 50 navy Havana SC fits you great, but when you order another one in a different colour it's suddenly too small.
Inconsistent sizing isn't just a problem within a suit line, it appears to be a problem within a single suit. In a hurry, and not wanting to spend either the money or the time on my usual MTM fair for a new linen suit, I ordered the Blue Check Lazio linen 3 piece suit (46L) ( http://us.suitsupply.com/suits/suit-blue-check-lazio-p3548i/P3548I,en_US,pd.html?start=14&cgid=Suits ). I got the suit today, and tried it on - the experience went something like this:
Pants: Surprisingly, perfect; the rare slender and tapered fit that drapes nicely - just needs to be hemmed.
Vest: Pretty darn good - no alterations needed.
Jacket: Sleeves, perfect; length - a bit short by my standards but not bad; then I tried to button it - wtf?! It was too tight when it should be a full size and a half larger than my actual measurements (I bought the size up for sleeve length, not chest and waist dimensions). So I got out my tape measure and, sure enough, the vest is right, but the jacket is simply not cut correctly. I believe it was intended to be the indicated size given that all other measurements match the SS sizing guide exactly and if it followed the sizing ratio properly it should fit me with room to spare.
It's too bad because, for the money, it really is a quite a nice suit. The linen is really terrific, the fusing is lightweight, the half canvas gives it nice lapel roll and drape that, from a few feet away, make it look like a more expensive suit. Unlike a lot of linen suits I have looked at lately, it doesn't feel over constructed and heavy. It's also disappointing because it fits well in most other regards. I am still trying to make up my mind on whether I'll return it outright or try an exchange.