Spoke to a rather knowledgeable person just now regarding Sat Navs for the Golf.
He supplies and fits new RNS 510, Kenwood and Pioneer Sat Navs to VWs, Audis, etc
His take:
RNS 510 is superior on radio and the aftermarket ones are better on CD, DVD etc.
Radio reception from the aftermarket Sat Navs can be very poor even with boosters. This is due to arial incompatibility.