Few cents worth:
Can the stock handling be improved? It is an absolute yes, with qualifiers
What one calls "good handling" may be another's nightmare. One has to first determine what the objective for a suspension modification is and see if there are solutions that can get them there.
For example, if the purpose is to just have a lower look, you have to decide how much lower, what spring rates will accomplish that and see if the stock shocks can handle this without revalving. My rule of thumb is that if you lower any car beyond 1.0 in. and want to ride on the street with that new height, you should seriously look in to appropriate coilovers or revalving the shocks if they can be.
If the objective is actually "good handling" then the equation is much different. Spring rates are only part of the equation as is the lower center of gravity and actually the mean center of gravity. This actually requires truly complex calculations and "good handling" has to be precisely defined by the owner before any such calculations.
Third, if the purpose is to get in to competition, such as autoxes (solo II) or solo I, then the first thing is to look in to the rule book for various classes the car should be prep'd to.
I know I am not giving you specific answers, but that is actually my point. Many tuners do focus on engine tuning and claim to have matered chassis tuning, but frankly, I am a little cautious about those statements.