I've found that 155's tend to be biased towards understeer. I you wanted to dial in a bit more oversteer.........
Disconnect one of the anti roll bar drop links on the front bar. Very lairy cornering guarenteed.
Put a thicker roll bar on the back
Soften the front springs
Stiffen the rear springs
Basicly what you are trying to acheive is a greater transfer of weight / inertia to the front of the car. This will make the front grip harder than the rear and cause the back end to slide more readily. Definately more fun but not something you want on a motorway sliproad in the dark, in the rain. (hello armco, havn't seen you for a while).
Generally lowering a car will inprove grip though the elimination of body roll but will introduce more understeer into the equation as a trade off because there is less front/rear weight transfer. I found that this wasn't the case when i lowered mine with eibach springs. I think this was because lowering a car also alters the suspension geometry, on the 155 it introduced a lot of negative camber on the front wheels. Which in turn increased front end grip. Hence less understeer.
If you really want oversteer, buy a 205GTi or an E30 BMW M3. Both will teach you the art of going round a roundabout sideways and introduce you to their best mates Harry Hedge and Derek Ditch.