I wish I could agree with you Par, but unfortunatly I've already found out through one of our clients and also through our own testing that this doesn't hold.
One of our clients who was especially chasing a light weight dinghy, asked if I could recalculate one of our designs using the suppliers figures. Using 460kg/m^3, the boat should have come out to approx 40 kg. So, he rang the supplier and purchased the plywood based on the suppliers figures for weight.
After building the boat, I asked him to weigh it and to our surprise it came out to 65 kg. The builder went over every part of the boat, trying to figure out where the extra 25 kg came from. So, I opened up my weights and moments spreadsheet, I recalculated the boat using 600 kg/m^3 plywood density and the spreadsheet result was 64.5 kg boat weight.
Having some samples of this Lloyds plywood, I then did a density test and yes, the result was ...600kg/m^3. Further testing resulted in
6mm plywood = 600 kg/m^3
9mm plywood = 540 kg/m^3
Overall, through my own research, testing and use of Lloyds Special Service Craft standard, I could say a lot on this subject, particulary in regards to this "approved" plywood and all it's so called marketing, but I feel that I must leave it all for obvious reasons... unsaid.
I couldn't agree with you more mate