i have a 55 reg 156 veloce and a 55 reg GT(both JTD), the GT has Eibach springs and Koni dampers, 3.2 rear roll bar(best and simplest £ for £ mod), the 156 is standard,
I could not get the GT right as the front Eibachs were two low, they looked great but hit everything(exhaust,undertray etc) and the ride over bad surfaced put me off driving it, when it hit a depression i thought it would break something.
I eventually changed front springs for the 3.2 version, the petrol 2.0 and jtd take the same on a gt...but not always on 156 147 ??? (after talking to Eibach)as they are simply 15mm longer(but same rate) and now its perfect, it rides well and dont hit anything.
The 156 rides better but cant keep up on the twistys but as a daily drive its more comfortable (both 'have same wheels and tyres). the gt is great if you find an nice road, but so is the 156 but not quite the same.
If you want to improve looks then lower but if you get it wrong it will annoy the hell out of you, my only consideration with the 156 is possible spacers 10 -15mm max to fill arches ....it would look then look good with 18".
I can say for certain my veloce is lower that two other 156s arround me (usually ones without the side skirts lusso's i think they are) and i know it handles that little bit better.
Lowering/moding my gt was worth it as the arch gaps were huge, it now drives well but i changed the front 3 times to get it right (new front standard bilstein shocks could not cope with the rebound from eibach springs hence the second change to Koni sport over bumps it just bounced, i fitted them first as i was trying to avoid a harsh ride)
it also has 20mm spacers as when dropped your wheels seem to fit further into the wheel arch !! this altered the stance the most....
so it depends if you want looks or performance, and also do you want to be happy with the daily drive ?