It's all about the effect on the audience.....
 
For me... to demonstrate Superman's power, to wow an audience you must keep his feats relatable...faster than a speeding bullet....more powerful than a locomotive....are perfect descriptions as the comparison is real.......moving planets, flying thru the sun, etc. are all fine and dandy, but the audience has nothing to compare them to...I suppose you could have a scientist in the film detail the temerature of the sun or the force necessary to actually move a planet, but is the audience buying a ticket to a science lesson?........the film should focus on relatable feats, and to do that you don't need Superman at a planet pushing, solar swimming, and hence somewhat unbelievable, level.
 
 
It's like the star ship Enterprise, it's hauling it at warp 9, and crosses the immense void between star systems in weeks or months, but is that speed ever relatable.