The RCBS version is still substantialy improved just looks like it should feed better. The 25-06 works fine but is a bit long for my liking in a standard mauser action.

reading from my Hornady manual:
.257 Roberts Improved....117 grain bullet...max load RL19, 49.6 grains, velocity of 2900'/sec with a 24" 1-10 twist barrel.

Read into it as you wish.

Frank: I've always used the rule of thumb that X- increase in powder results in X- increase in pressure and 1/2X- increase in velocity. However, this is in the same volume case. The 1/4- rule of thumb for increasing case capacity would seem a little conservative, but not much.

I don't know the actual numbers, but since the .257 Roberts has a bit more case taper than the .280, the AI treatment should result in a greater percentage gain in case capacity in the .257. At any rate, the .257 AI has been around for many years and has always been regarded as one of the best "returns" on effort of any of the AI series. It's always puzzled me that the 6.5-, 7-, and 8 x 57's (made on the identical case) aren't more often improved.

Not surprisingly, the greatly improved velocities that many shooters report when "improving" a chamber are often the result of the shooter loading the case to higher pressures in attempting to reach pre-conceived notions of improved velocity. That's not to say that "improved" cartridges are not useful and in many ways superior to the parent, it's just to say that "improving" a chamber is a marginal way to get increased velocities.