Wickahoney, Your post was very informative. I don't know if I can add anything other than I also love the .257 Roberts. I only have owned one rifle in this caliber and it is a Winchester Mdl. 70 Featherweight purchased new by me in 1981. I have tried many different loads using all the IMR powders you mentioned and the load I found best for me, in my rifle, for whitetails is a 100 gr. Sierra with 44 gr. of IMR 4350 with a Federal standard primer. This load will group 5 in an inch and a half at a hundred. Not a target rifle for sure, but it does the job. It has taken at least one deer per year since 1981in PA and WV. It will shoot a little better with an 87 gr. Sierra in front of the same load, but i don't really trust the 87 for deer.

I'm sure in other rifles where you can seat the bullet out to the lands it will shoot a lot better than mine, but even with it's little quirks I really love this old caliber.

Good luck and have fun experimenting with loads for your rifle. You just might find that one special load that will really group great.

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Big Fred