In the 6mm class, the 243Win is so close to the top of the heap that nothing beats it by a margin wide enough to offset it's availability and decades of proven performance, IMO, except the WSSM.
No other 6mm cartridge beats the 243Win by more than roughly 1-1.5% velocity. 50 fps just isn't worth the trade offs of stepping away from one of the most common cartridges in America.
The WSSM beats the Win by 200fps or more (about 5%), bringing 55gr bullets up to some of the fastest speeds of ANY cartridge with ANY bullet. It matches .204 Ruger 32gr speeds with a bullet 71% heavier.
For me, that makes it worth it.
The other one that's amazing is the 223wssm. Published velocities with 35gr bullet at 4,600+ fps.
Another factory dead marvel.
I can't believe how quickly they abandoned them either. They weren't even introduced until 2004 and they're long since gone. You'd think you'd give them more than a couple years. Now some of the guns are selling for more than they did new. There must be some demand.