I think the answer to your velocity question is 308, but all are excellent calibers. If you want one of the new 7x57s you had better act fast as they may already be sold out. I have (for sale actually to finance my incoming 7x57) an RSI 30-06. I have never felt there was a problem with the velocity loss. I just needed an RSI in 7x57, so Lipsey's came through for me this year. Great gun in a great caliber. I personally would want a longer barrel on a 270, but that is just personal preference. If I could afford an RSI in every caliber they make I would get them all. To me it is the best looking rifle available.
Not to hijack your thread, but the International isn't on Ruger's website this year. Are they still producing cataloged M77 Internationals? Did I miss something?