fwiw, my last boat had the ex343 in it. with a 1235 prop. home lake- lucky peak reservior- is at over 5500ft. granted we don't board but surf 1600lbs on surf side and 400 up front with 6 or 8 in the boat and never struggled.
current boat is a 23.6ft pig and loading up with 1600lbs water, 500 lead, 500 in human ballast and 800 water up front. same prop and mead is at 1300ft never even skips a beat.

imo prop selection is just as if not more important than motor selection. I would not hesitate with the 400 but you might want to swap out props.