Well they make the cars better for two reasons, the damper quality is leagues ahead of OEM so the ride is barely impaired, and the springs have a much higher poundage so grip improves. The problem for a family car will be on the motorway. Youll have less travel and it wouldnt be ideal for getting a baby to sleep!
A good way of reducing understeer is getting an anti roll bar kit. Get a front and back ARB because the Megane isnt exactly light at the front. A thicker rear ARB will dial out some understeer. The good thing is, you wont notice a ride difference.
You might want to get an Eibach spring kit which will lower the car slightly. Obviously lowering the CoG is a good thing, and the harder you go round a corner the stiffer the progressive spring will be become. As many Cliosporters found though, being progressive the spring starts soft and some people even think the ride is better with them!
-Rob