The biggest shock for you would be how stable the car is at high speed, 100mph on motorway feel like 70-80.
as for the JTD, you might find the car understeer badly as some inexperance member think and start saying the stilo handling badly. you just have to adapt your driving style a bit.
You should get superb grip on thosemine has "16 205s and it understeers badly and well if the road is wet, it´s a nightmare, i´m pretty sure something has to be wrong with it : (
I really don´t know i mean the car is barely 5 months old, so the tyres don´t have that much use on them, and they aren´t exactly the cheapest nastiest tyres, they are continentals R16 205/55 96V, so i´m really thinking of going to fiat and ask them to have a look.