I remember discovering the de-tuning thing years ago while researching MD trucks; IIRC even some class 5&6 trucks run tunes in the 200-240hp & 500-600tq range.
If you look at MD & HD build sheets, there are usually multiple power levels to choose from, with quite a broad range.
As already pointed out, reliability & duty cycle expectations go up and there's still enough performance to get the job done at reasonable speeds. An inner-city delivery van that'll never tow and rarely even see the freeway just has no need for 350hp.
