The 9.3x66 might beat the 9.3x62 by 1% or so, when loaded to the same pressure.
I totally agree with Mule Deer ! The velocity gain is perhaps 1%.....the main reason is that magazine length and COAL is exactly the same for 9.3x62 and 9.3x66 so the bullet only goes deeper in the case with the 9.3x66........here is a pick that explains it very well with 286 gr bullet. So punching the x62 out to x66 is just a waste of money !
This is the tiny area for more powder in the x66 case !
The 9.3x66(left) & 9.3x62(right) :