Thanks. I did that "sail efficiency test two times only. One time I was on a slalom board (iS76) 6.6 was OK. Only ever so slightly on the big side I still wanted to try my 5.5 which was very underpowered, changed fins from 26 slalom to 23 speed too. Still went pretty much as fast and the biggest difference was that it was hard to get in position for a god run with the smaller gear.
A few week ago I was sailing with my wife. She was on 4.0 and I one 5.0. Lake sailing but wave sails. Kombat 79 for me. 5.0 was definitely on the overpowered side but 4.0 was still only reasonable powered up for blasting purposes. I still went around 1.5-2 knots faster with the smaller sail (and a 100 times more comfortable).
In both these cases, 2s speed readings was around 30 knots, I think.
But of course there are instances where a bigger more powered up sail would be faster.
The final answer on "weight dependance" is that it depends...