Kastaway,
I think where all this weight/HP thing falls down is when insurance companies get involved. Hulls have previously been designed around HP which has been a structual issue ie only built to carry x amount of HP. Now 4 strokes have landed on the market manufacturers have had to include maximum weight.
I have been thru this with a manufacturer who re issued a plate for a particular boat to carry the extra HP this covered the owner for insurrance purposes, otherwise in any accident that was investigated propperly he would have been liable.
Garry