Do you have a Check Engine light too? I forget the code that was showing up on mine.
Chances are this could be the "purge valve". I don't know if this effects the XG350 but i have a 2002 elantra and i have the same problem where it is very slow to fill up. It can take me 10+ minutes to put 40 litres in and i'm not looking forward to doing that when it's -30c this winter.
The quoted price to get it fixed was around the $200-$250 cdn mark.
Chances are the same model year shares some similar parts.. apparently the purge valve sticks open and the new one is a different design. In my opinion stuff like this is a design fault which they have perhaps should be calssified as a recall to those who are effected by the problem.
Why is it these days we seem to get plagued by issues which should be none issues... We all expect to change some items but we seem to get stranger problems all the time. I know cars keep evloving and getting more complicated but some things shouldn't really break during the lifetime of a vehicle.