You have a great rifle in a wonderful caliber. The roberts was always down loaded to a ridiculous level. Even the +P isn't up therre with other rounds made in the same rifle such as the 264 270 and even the 06. I had a Pre 64 in 257 AI many years ago and I loaded it to the base of the bullet(any bullet) with the late great Surplus 4831. That rifle shot great. I was cleaning out the other day and found 250 257 AI cases and another 700 7mm AI cases that I had fireformed with cream of wheat and to feed the birds. Then someone came along and all but stole that Ruger 77 away from me. It also shot great. Better than the original 7mm Mauser for sure. Enjoy that M-70
I didn't know it was new again, but I've shot some +P .257 Roberts Hornady marketed about 20 years ago in a 1947 Winchester Model 70 .257 Roberts and the rifle shrugged it off.
It's certain that some handloaded concoctions here have bested factory so called +P .257 Roberts anyway.
I wouldn't worry about that one in the least. Love the .257 Roberts; I've owned two, a Ruger 77 and a No. 1. Both great shooters. Handloaded 115 gr partition Noslers are dynamite on deer.
To echo other's, as long as the rifle is in good condition(properly headspaced, etc) it'll handle anything produced or published by reputable handloading sources.
I own both .257's and 7X57 calibers. Most factory offerings especially at the time I got into to them were anemic at best.
The Bob and the .41 Rem Mag are the calibers that got me started in handloading!
I've got a Model 70 Featherweight, topped with a 4 power Luepold Compact that has been used by several first time deer hunters. Easy to carry, easy to shoot, and the recoil does not get them flinching when they are checking the zero before the hunt.
That Nosler 115 grain partition and IMR-4350 just flat work.
For 100-117 gr bullets, the velocity gain for +P is between 100-130 fps. If you did a back-to-back shot comparison between this and standard pressure I'd guess you'd have a difficult time telling which is which.
I recall many a story in Outdoor Life by Jack O'Connor about his wife Eleanor's use of her .257 Roberts with good results. Can't imagine Jack letting her use something that didn't work.