I'd be surprised if a blocked carbon vapour trap was causing your slow filling issue, I can't say where that vapour trap is located though, logic would suggest well forward in the RHS wheel arch area, though I don't remember seeing it when I had the wheel arch liners off a few months ago.
Edit:- more like the tank filling breather pipe has a problem.
The space above the fuel up to the filler cap will get vented when the cap is on, but if that was blocked it should not slow down the rate at which you can fill the tank from a fuel station nozzle - think about it, when you are filling the tank, the engine etc is switched off so the carbon fume can can't get opened and start purging as it will not be enabled.
Surely That's why the carbon can is so big so it has room till the engine is started and it purges. I'm just going on the fact that when I Google 'audi won't fill with fuel' all the responses point to the carbon can / evap system but I can't even look at it to rule it in or out till I know where it's hidden.
I could be wrong here, but I don't think that you are going to find any big carbon can any where in that car, there is only a small one as everyone else uses, there is simply no reason to have a large carbon can.
So, start looking a bit more, find the EVAP pipework under the bonnet, follow that to the EVAP solenoid, then follow that back to the carbon can which traps the fumes collected from the tank.
Edit:- it is even possible that the EVAP solenoid s plumbed directly into the inlet manifold area as it is a charged induction engine - that is where it is on my wife's 1.2TSI Polo engine.