Fieldofdreams- I fit perfectly in a size 34 Lazio with little to no alternation for the jacket, although the pant was a little tight, but still comfortable for me. Should I size down to a size 32 Napoli or stay at size 34? My body is slim, athletic. I have broad shoulder and a narrow waist at 5'6 140. I think my shoulders gives any suit jacket I wear a good structure fit as long the size is correct.
If you size down the pants will also be size smaller. So, my best guess is no, sizing down on a napoli will not work for you. Also, keep in mind that for me a 38r napoli is markedly slimmer than a 40r lazio. You can have the pants let out marginally. But if you size down an entire size that won't be enough. If you need more shoulder room I'd try the napoli in your same size.