The rear brakes on a FWD car only do 10% of the braking. So even if you double the stopping power of the rear brakes it only gives a minimal increase in the overall braking power of the car. Plus, better rear brakes cause all sort of brake balance problems so you will probably end up reducing (bias valves etc) the rear braking effect negating the effects of the upgrade anyway. Add the fact your handbrake will be next to useless with discs too. And!! Because the rears do so little the discs will rust up and the callipers will eventually seize from lack of use. So, what is the point?

The mechanics at my dad's Ford dealer can't stand rear discs as the spend most of their time unseizing them and replacing rusty parts because they do so little (braking) work. When cars with rear drums come in for a service all they needs is a 2 minute check and the shoes last for years and years.