Try enthusiast car insurance. RS4 & 6 with them. You can get agreed value and nominate how far you drive in a year. Premium is based on value and miles driven (amongst other things). NRMA used to have a good policy but they started putting severe mileage restrictions on it so I upped camp and moved.
I have never had a claim so frankly I cannot even begin to guess how any company would deal with it. I have used Shannons way back but lately they want too much info before giving an indicative quote. And of course it then goes into the never to be deleted records so your cars location is accessible to some people even though you have no business with that company. Not for me.
C5 RS6: Sportec, Milltek & Wagner -- B5 RS4 : modified -- B5 Audi 1.8 quattro 132 Kw -- 2007 VW Touareg TDI 3.0 with air suspension and lockers-- UNIMOG 404 TLF8