When properly designed, and the pisiton sizes are appropriately sized, you don't need a FR/RR set, other than for aesthetic reasons in many cases. You don't hear of folks overheating the rears on the track, or eating wheel bearings due to heat.
Often more pistons = means smaller pistons. But overall piston area compared to stock could be the same, or slightly less, often shifting brake bias rearward. Which if you think about it, is a good thing, as it can make straightline braking more of a planted feel, vs a front dive under hard braking.
The other benefit though to a true FR/RR kit, could be as simple as faster pad changes at the track.
Either way, you'll have something that can consistently perform better under temp, and look wicked cool too. Not to mention a very real added benefit of less unspring weight.
Funny...I looked at the AP's, and Brembo GT's (and the RS peace meal option), AND Stop Tech. All offer pretty bad ass options............for the front ONLY! So aggravating. Baer doesn't offer anything for the Focus. Wilwood is the only one with a four corner option. This alone knocked everyone out of contention for first. And if you're not first....
If I'm being completely and truly honest, the Wilwood's were my favorite looking rotors and least favorite calipers. But, doing only the front and leaving the back the way it is (or any existing options that retained the stock caliper) coupled with a gorgeous front just wasn't an option for me.
And the Wilwood is an amazingly well thought out kit. There's no piecing anything together. Everything is there in one box and made to fit. Bravo. That fact carried a lot of weight too.