I'm afraid I must disagree.....
As I said above, on the most recent software, the icon in the top left is not shown as large as it used to be, but the "X2" icon DOES get used. (or at least it does on all of my my TomToms).
To prove it, I have set up a warning for a pub on my XL, using my own POI file with an icon of a beer glass.
Here it is with just the normal sized icon installed:
Here it is with the normal sized icon AND a a 44x44 "X2" icon installed:
And here it is with the normal icon and a TOTALLY DIFFERENT picture installed as the "X2" pubs icon:
As you can see, in picture #1 the normal icon is scaled up slightly, to be used in the top left corner.
In picture #2, the 44x44 icon has been used instead.
In picture #3 the new 44x44 icon has been used, proving that it's not just the normal icon re-sized.
So... we need to work out why it's not giving you the results you expect...