Your tyres and tyre pressures will make a big difference. When I bought mine (1.8GT Blackline with 18's, so the lightest with same suspension set up as yours) it had a set of cheapo tyres on the front. It ended up being very skittish at the front and felt as if they were made of rock. I changed them for Pirelli P-Zero Rosso (as on the back) and it transformed the ride and grip. Yes it feels firm (but softer than my previous A3 on 17" Michelins) but it also feels compliant, and it just grips.
I know I sound like a Pirelli salesman (I'm not!), but make sure you have whatever you've got on set at the right pressures, and if it's still not right get some softer tyres. I think the manual actually specifies Michelin Pilot Sport or P-Zero Rosso only for the 18's.
I went from 17's to 18's in a 159 and made the ride softer by fitting Michelins and Pirellis instead of the Bridgestones it came with. Also went from 15's to 17's on my 147 without any adverse effect by making careful tyre choices.
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........