I think you've got to look at a couple of things:
- The value of the pub without an auto
- The value of the auto (say on an 8x10)
- The timing of the date of publication and how long an athlete had to sign it
I collect autographed Sports Illustrated magazines and in virtually all cases, an authentic autograph will increase the value of the magazine.
In this case, the value of the unautographed magazine is probably an order of magnitude higher than an Eddie Matthews autograph. I would guess there will be a small premium, but not much.
One of the reasons that I like the signed magazines is because they are only published for a short time, versus an 8x10 glossy or some other photo that could have been produced without limit over many, many years. So while it does not date the auto, it does date the medium.
If this were for a signed Mickey Mantle SI from 1956, the autographed copy would bring a strong premium over an unsigned copy. Note, not mine - from Mears 2013 auction