Well, final test, I tried with a real movie.
Just converted a chapter of a DVD with Handbrake. I chose a movie with some grainy pictures (Happy Together - Wong Kar Waï). Same settings than before, 576x352 was perfectly fitting the movie without any cropping.
And the result is ...
...
...
Very good !
Not jerky, not blocky ... just nice !

Frankly 576x352 is a loooooot better than 400x240.

Can somebody try and confirm these findings ?