Deako,
Eddie-NL has done this very mod on mine! It's a bummer but you just need the BT kit and the DAB version of the RNS-510. You can sell the 315 to off set some of the cost but it's well worth it.
I used the opportunity to get the latest version that has an ssd instead of the normal hard drive but I'm convinced the amp inside is an improved version over the standard "C" version I had in the Golf, as it sounds much better.