To get down to say 750kg, you don't get something for nothing. Short of building every bit of the whole ship out of carbon/kevlar (which would make it cost £200k+) .
Shome mishtake shirley?
My 1948 Luscombe has a MAUW of 1400 lb and as far as I'm aware carbon/kevlar hadn't been invented when it was built.
450kg perhaps?
I agree with you, the paring down of what in the marine world are called scantlings to get under an arbitrary weight limit is worrying. Yes, the stress calcs will work and the test airframe will pass the load tests but you end up with something like the performance figures.
A test pilot in a brand new aircraft will be able to achieve a certain performance. That doesn't mean that Joa Soap in a twenty year old one can do the same.
In the smae way the fact that a carefully constructed prototype passed the load tests does not necessarily mean that every example thereafter is built to the same standard or that its strength several years later is the same as when new. You need to allow a bit of leeway for variations in build quality.