Lott outperforms Win
Since one can also shoot .458 Win in a .458 Lott chamber, reaming your chamber to Lott factually gives you more options. Fact.
Factually too, whatever can be accomplished with a .458 Win case can be bettered with a .458 Lott. Again: fact.
Whether you need what the Lott can do above what the Win can do is mostly a matter of ... opinion, although, factually, the Lott does fly faster and flatter.
Lott generally outfeeds Win
Feeding reliability is fundamentally the product of a proper relation between cartridge shape (i.e. taper), cartridge diameter, magazine box width, and feeding rails profile. This is the reason why Mauser made, and top notch gunsmith still make, different magazine box sizes for different cartridges.
This being essentially a lost art, a longer case of same diameter gives a little more control to the magazine rails during the feeding process. Hence a fat, straight wall, short case like the Win generally has more feeding trouble potential than a similar but longer case like the Lott.
However, in some actions where the magazine box is technically too narrow for the straight wall .458, because they were designed for cases with significant taper, the only way manufacturers can make the .458 Win to chamber OK is by making significant relief cuts in the first third of the feeding rails from the chamber. An unintended consequence for this quick fix, is that it may compromise a Lott case from feeding well. In my experience, this is not the case with the CZ which generally feeds well with the .375 H&H, .404, and .416 Rigby case heads. Another story when it comes to .500 and .505 case heads as many CZ550 1/2" bore owners discovered to their dismay...
The bottom line is that pushing a reamer through the chamber is not even half of a proper conversion from Win to Lott. The real work is in insuring reliable feeding of both Win and Lott cases off the magazine box. In some cases nothing is needed, in some cases some time-consuming, but not necessarily difficult, tuning is needed.
