Alright fella
i've just replaced my old RCD300 with the RCD310 after the old unit decided to turn itself on whenever it felt like it!!
Anyway, the 310 came from a 2010 scirocco. Then got a canbus from a different, but same year/car.
Got given a link on a how to guide for the canbus
http://www.my-gti.com/2296/can-bus-gateway-removing-installing-and-upgrading-volkswagen-golfthen stumbled on another with i found more helpful
http://www.my-gti.com/1101/upgrading-2nd-generation-radios-rcd-300-rcd-500-to-3rd-generation-rcd-210-rcd-310-rns-310-rcd-510-power-drain-issueHowever, with regards to the canbus and its battery drain issue, i have a 1k0 907 530J fitted and programmed in. Had it installed for the best part of 2/3 months now with no issues, (touch wood). As long as you can get someone who knows what they are doing, then i reckon it should be fine. The guy who did mine was telling me the ins and outs, like having to disable things like the parking sensors, electronic suspension, etc. Took him a while to do, then tested the system for faults, which nothing was found.
As for your areial, you can get an adapter for the two different connections off ebay for about a tenner. My 2004 GTI has no roof antena, and work perfectly fine.