Well, i'm not talking about chavvy upgrades. There are very good reputable upgrades for Alfas of all kinds. They don't need a lot doing to them, but a lowered and stiffened suspension set, uprated anti-roll bars and some strut braces can increase overall chassis rigidity and increase tyre contact with the road when set up correctly.
It's generally considered as improving, not spoiling.
As for a newer 156, as someone has already pointed out, the most powerful you can get is the 2.0 JTS, with a supposed 165bhp, however there have been claims that people have had them rolling-road tested with as little as 120bhp. The only other way is to go with the 1.9 JTDM and have it mapped, etc, but you're not wanting a diesel.
There really is little choice, but if you want my personal opinion - go for a GTA, if you can find a good one.
Otherwise, you're into 159 terriroty.