Hey fella, did swapping the starter solve your issue with sluggish start? and was the sluggish start worse in the cold? I did some digging around and the draw should not be over 180a when engaging starter, excuse my ignorance you said you measured 400amps, how?
If you battery checks out and charging system too then that leaves the starter I guess...
3.0d battery/slow starting problem?? Read on pls