What Pascs said, the 156 is most limited by its suspension and braking not lack of power.
However, if your 2.5 is going to be a motorway cruiser and handling is less of an issue, get the 3.0 intake runners (Chris 155 is selling some on here:
http://www.alfaowner.com/Forum/parts...nd-plates.html (3.0 / 3.2 Intake Runners and plates) ), a decent filter system (I use and thoroughly recommend BMC's carbon dynamic airbox) and then take it to Red Dot for a custom remap.