I just finished an 8.5x57 (.338"x57) wildcat. Very impressive. I like it so much, I sold my .338-06.
I can't imagine what a .338x57 could possible do that an 8x57 couldn't, but that's just me...
I always liked the idea of the 9x57, but using .358 barrel and bullets instead of .355 or .356.......just because of the better bullet options with modern ammo.
either way, the round falls between 358 Winchester and the Colonel's 35.
I'm on my 2nd 9x57 at the moment, and am building a 3rd. The 1st one was a rebored 8x57. It had a .355 groove dia. But .357 bullets worked fine with no indication of excessive pressure. But since I have a quantity of factory 9x57 Kynoch ammo, (More than I'll ever use in my lifetime...) I'm not much inclined to worry about handloading. My current 9x57 is a M-78 Remington. (M-700) It has a tight chamber, but will take the factory rounds just fine. It is an american barrel, so I assume it is a .357-.358, but it shoots the factory ammo as fine as I can with a recv'r sight. And I can personally attest that it puts down deer with aplomb...
GH