If you handload, the 7x57 will be virtually indistinguishable from the 7-08. Case capacities are very close - the 7x57 actually holds about 2 grains more water - so performance will be identical for any practical purpose.
Since the 7x57 barrel will likely be custom and cost more, is this really the most economical route? The only difference in loading for the 7-08 will be a set of dies (all else is the same - powder, bullets, primers, shell holder), and decent dies can be purchashed for under $30. I'm certain that the premium for a custom barrel versus a factory one will be more than $30.