Other considerations...
Old Southpaw,
I was considering having a sweet little Browning BLR rechambered to 257 AI a couple of years ago, and did a lot of research on the subject. All of the opinions stated here are valid. As many have implied, it just depends upon what means the most to you. I decided not to re-chamber, and here are the factors that influenced me, besides the fact that I would have ruined the resale value of my rifle. First, as several people have mentioned, not all AI conversions are equal, in terms of improved performance. The 250 Savage AI is one of the two best, with approximately a 17% increase in velocity. The improvement to the Roberts was meager, as several have pointed out. It was already a wildcat, and very efficient, before becoming standardized. Something that nobody else has mentioned is that SAAMI has increased the maximum pressure for the Roberts from 45000 CUP to 50000 CUP (referred to as a +P load, Winchester makes specific +P brass, but standard Remington brass has not exhibited any problems). THAT makes a substantial difference in the performance that can be wrung out of the standard Roberts cartridge. My BLR has a 22" barrel (from factory, long story). I have chronographed my 120 gr load at an average of just over 3000 FPS, and my 75 gr varmint load at an average of 3735 FPS, with no pressure signs, using Reloader 17 powder. That's some mighty snappy performance from what some consider an "obsolete" cartridge (My "Bob" has become my absolute favorite rifle). The Reloader 17 powder plays no small part. It is intended for use in smaller capacity cases, like the Roberts, and the short magnums. I have been playing with it quite a bit in my compact magnum cartridges too. It typically provides 150-200 FPS more velocity than any other powder that I have tried, with no pressure indications, and excellent consistency/accuracy. Loading data for Rel17 is still sparse. You will need to be creative and cautious in developing loads. Some of the more popular reloading manuals are now publishing +P data, and Rel17 data should become more widely available soon. I use the "Load From A Disk" software, along with other references, and am very conservative in developing a starting point for my test firing ladders. I echo the sentiments of most of the other people on this thread. Try your Roberts with standard or +P loads before modifying it. The 257 Roberts cartridge is a honey! If you do decide to rechamber your Ruger, be certain which reamer and dies you use. RCBS developed an "improved" version of the cartridge too, but it is their own configuration, not Ackley's, with a 28 degree shoulder instead of the 40 degree Ackley shoulder. Just be certain of which reamer and dies you are buying. Hope you enjoy your "Bob" as much as I do. (If you really want to play with an AI wildcat, take a look at what an "improved" cartridge does for the 250 Savage. It's impressive.)