Okay, first of all, if you're a .303 British nut, you really NEED Steve Redgwell's books:
Shooting and Reloading The 303 British and the 303 Epps and
The Accurate Lee Enfield.
Steve's a good guy, and his books are a great source of info on the cartridges and the rifles. Get them here:
http://www.303british.com/As to the .308" version of the .303British, take a look at the .30-40 Krag as compared to the .303 and see what you think.
Here's the .303 British:
And, here's the .30-40 Krag:
Hmmm....
As to the .257" wildcat off the .303 British, pages 92 and 93 of Redgwell's
The Accurate Lee Enfield list the .25/303 British and the .25/303 Epps. The .25/303B will give you performance between the .250 Savage and .257 Roberts, and that's not a bad ballpark to be playing in. The .25/303Epps gets you to .257BobAI levels in the P14 or a good single-shot, and that ain't shabby, either.