Load ratings are the biting bug bit. A 159 1.9JTDM maximum vehicle weight is 3.55 tonnes - have a look at your V5 or the manufacturers plate under the bonnet. Start going down load ratings without getting your insurers agreement and you could end up in a bit of a pickle.
Secondly, as the 17" wheel tyre combo is the most accurate although not 100%, 19" wheel combo has a smaller rolling radius to start off with (1% fast I understand) start adding in the 2.79% as well and you'll be 3.79% fast.
Thirdly Alfa designed the car and spent millions on testing different specifications to ensure the best overall compromise why do you think that fitting something different will improve the situation? Surely a wider tyre known for it's poor wet performance is more likely to stick you in the hedge than the one it is specced to have?
I am very open to your persuassion.
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........