Hi Malcyb
This can be done on a Mk6 (not sure on a Mk5), as last year I finally upgraded the RNS510 headunit in my 2009 Mk6 GTI that has Dynaudio fitted. I went with a Pioneer AVIC-Z930DAB unit. This Sat Nav unit has wireless Apple CarPlay (and Android Auto), plus with the correct harness adapter it retains the displays for parking sensors, rear view camera and climate controls/heated seats on the head unit. Albeit Pioneer's graphical versions of these.
You will lose the "Audio" and "Navigation" information on the car's DIS screen (between your rev counter and speedo) though as I couldn't find a unit that retained that. I believe it's sent over the car's CANBUS network from the headunit and I guess it may be only available via VW's own headunits?
Anyway the tricky bit was getting the correct aftermarket harnesses and leads to connect the headunit to the car and be compatible with the Dynaudio, steering wheel controls and rear view camera etc. I sent off a few emails to Pioneer tech support and Connects2.com about what products were compatible.
Also I found a VW document that mentioned the way that the Dynaudio amplifier works in the Mk6 and that it has a speaker level of minus 14 dB on each speaker channel.
"The signal inputs on the amplifier are controlled via
the speaker outputs on the radio unit or radio-navigation
unit.
The radio/RNS unit needs to be coded to “Sound” so
that the level of the output channels is reduced by
14 dB."
So I applied this to my speaker level on the Pioneer AVIC-Z930DAB unit. This cut down the amount of background hiss with no audio source dramatically, plus I guess if you left it at 0 dB you may blow the car's speakers if you have the volume too loud?
I can post what my shopping list was at the time if that helps if you decide to get a Mk6?
Also at the time I upgraded the car's GPS only sharkfin aerial to one that has DAB/FM/GPS on it, along with a Pioneer accessory that converts the VW Multi-Device Interface (MDI) in the centre console (under the arm rest) to a standard USB connection rather than VW's propriety connection.
Cheers
Bluto