I would think that the davit capacity shown is the max usable davit capacity. There is no mention in the label that the capacity depends on sea state, weather
conditions, etc. I have to believe that the manufacturer factors in a safety
margin (similar to chain SWL and breaking strength) to account for more extreme conditions. If not, things will be breaking left and right.
In other words, if you stay within the posted capacity, you should be fine, or you have a valid warranty claim.
If you do not believe, or agree, with the manufacturer's label, the manufacturer is the best entity to answer your question. People can argue all day but who can you really trust?
I have the AB 11 AL (172 lbs) with factory bow aluminum storage
(? lbs), dealer installed shorter & lighter fiberglass
console and seat (? lbs), and a 20hp Honda outboard
(110 lbs), 3 gallon fuel tank
(25 lbs full), small battery
& other minor stuff. So far, so good. I am guessing that the storage
and console/seat weigh around 70 lbs. By the way, 11'6" should be the max dinghy length for the 450, in my opinion.
P.S. I've had a couple of glasses of wine and I am tired...