As you comment elsewhere, better to take the advice of experts, unless they are proven wrong, and Berman and Paarmen claim the manufactured and launched Balance 526 weighs in at 9.3T.
Now, this is not out of the realm of possibility as the O51 is nominally 10T but Francois told me that a carbon bimini
and a carbon mast
will save over half a tonne in weight. The first Balance 526 has both. It also has all furniture fabricated out of lightweight materials.
Obviously this comes at an expense and I would hate to think how much that first Balance 526 cost the owner, but it would have been North of U$2 million I would think.
Similarly if you want a lightweight O51 this can be accommodated but the costs really mount up.
These weights are achievable but at considerable expense.