BMW/MINI doesn't make it easy for you to upgrade, and they sure as heck weren't kind enough to give you RCA outputs on the back of the stock head unit. Which really wouldn't be too much to ask since they made it extremely difficult to replace the factory head unit.
By the time you add an amp, wiring, and mounting hardware to that Pioneer kit, you're going to be significantly closer cost-wise to our Subwoofer System for the MINI. And you'll still have to figure out where/how to mount everything, and most important you'd have a system that doesn't perform nearly as well or look nearly as good as ours. Our 1101S system also provides a foundation for our upcoming Soundstage upgrade.
If our system isn't what you are looking for, no worries - you'll find our installation instructions (under the "Installation" tab) will help you navigate doing the installation on your own.