The 9.3x66 might beat the 9.3x62 by 1% or so, when loaded to the same pressure.
The 9.3x66 was primarily developed so factory ammo could be loaded to 60,000+ psi, thus providing handloaded 9.3x62 ballistics to people who don't handload. A pretty small market niche, but no doubt bigger in Europe than here.
Can't load factory 9.3x62 ammo to that sort of pressure (though a few companies load it a little hotter than others) because of really old rifles that might not handle it.
