Hello Mr Green, I fitted 2 aftermarket VW fans sourced via autodoc.com, part no:7022488
https://www.autodoc.co.uk/nrf/7022488. You are right, the fan blades have a much bigger surface area than the originals, so tend to restrict air flow a little, but I assume they just keep spinning with the force of air before the automatic thermostat switches them on. Currently on another long European trip and so far so good, they are doing their job well.