No, there is no rear caliber set from Spoon. Just front.
Just upgrade to their rear rotor and steel brake lines with better pads.

 

Too much rear breaking can lock up your rear tires while your front tires are still rolling, causing your car to spin since your front tires still have some traction while your rear tires have none.

Example would be riding a bike or a motorcycle. If you remember as a kid when you pedal backwards it brakes the rear tire. And you would stop in a cool fashion leaving a big skid mark with your rear tire and ending up with a cool sideways fish tail. But then you'd be cool and do that in the rain and wind up on your ass. Kinda the same deal.

For a better example just yank on your ebrake.