Unfortunately my insuarnce only covers ICE up to £1250 although I think if it ever got stolen I'd go with something aftermarket anyway, preferably with better unit security (removable panel etc)
Do you have other ICE aswell, or are you just thinking that a new RNS from VW would be over £1250? Because you could get two of them off ebay for that price.
I dont think the RCD front over an RNS would work to be honest. The 2 knobs on the front of the RNS would mean that the RCD couldnt get anywhere near flush enough, and if anything it would look like the stereo was sitting out- ready to steal.