Hello,
I have the same situation with my 2014 roadster. Holding the nozzle upside down at a 2 o'clock position helps a lot but does not fix the slow filling issue. The maximum capacity of the tank seems to be around 40 liters, if you try further filling, it will just overflow. Maybe it is limited by the way the nozzle goes into the tank leaving some air-room or the build in equipment like pump and filter uses the rest of the volume. who knows?
Can those who have managed to fill up more than 40 liter please report?
Regards
