If you have a perfectly setup car, which has neutral cornering, and upgrade the standard open diff to anything that locks (either a bit or 100%) like an LSD, viscous, mechanical locking, lockers, etc, you will then have a car which will understeer compared to before. Basically you are losing the differential action and therefore cannot have a speed difference as effectively as an open diff.

This is purely from a handling/dynamics point of view, as with all things theres another 10 variables that come into it, like grip which is basically the reason to go for these types of diffs.

J_Mech