Once you're happy with the powerflex upgrade you need to consider the springs and shocks. These are not up to much anyway, rear shocks in particular will give a jittery ride. Best combo is Eibach prokit springs (which only drop GTV/Spiders by 20mm) and Koni sport adjustables. Now this really will up the stakes and you will notice a quantum leap in sure-footed ride and handling.
And don't forget the strutbrace - Spider owners as well. Another huge leap in stability improvement for very little time and money. Can't understand why anyone with any 916 Alfa wouldn't fit one.
So in summary, for a sure-footed, confidence-inspiring AND comfortable ride, do:
1) Keep standard front wishbones and droplinks, but keep them fresh.
2) Powerflex the rear.
3) Fit Eibach Prokit springs (20mm drop).
4) Fit Koni Sports adjustables. Keep the rears on the softest setting and play with the front to see what suits you. For GTV's Koni themselves advise one half turn at the front.
5) Fit strutbrace.
6) For V6 owners if you want to use Eibachs purchase TS rear cones/bumpstops as they are better than the V6 ones (honest).