If you are not cropping, the standard 20MP image would give you about 9 pixels per mm or about 225 DPI.
The high resolution image would give about double. (all approx, as it would depend a bit on whether you printed to the long side or short side)
Unless you looked very closely or with a magnifying glass, you probably wouldn't see much difference.
The difference may not even show depending on things like the lens, atmospherics, how rock solid the camera was mounted, printer etc.
With perfect conditions, the high res mode would allow you to print 1/4 of the area of the original to the same standard as the whole frame of a 20MP shot.
On the screen it would only be obvious if you enlarged a particular portion until you started to lose detail, then looked at the same portion at the same screen size on the hi res shot.
Alternatively choose a section of a 100% crop on the hi res version, then make the same section the same size on the screen from the 20MP shot, and theoretically you should see a difference.
Whether it will improve an A2 print, viewed from a distance where you can see the whole image is probably questionable, even if every stage and component of the operation is perfect.
And even if it did, it won't make a poor shot look good, the photographers skills and talent will have a far greater effect on the "look" of the print