I am not a ballistician but I doubt adding 2" to a 458 over the 22" will matter much at all. The case is powder limited, so no matter what the barrel length its peak velocity is also limited.

I speculate that this is also true with my 416 Taylor. I get the same velocity out of my 22" barrel that others get from their 24" barrels. The case can only hold so much powder and hence in a relative sense can only achieve a certain limiting velocity regardless of the barrel length.

Velocity in a 458 is probably more dependent upon powder choice than barrel length.