Here is what I know.

Diffs work well when both wheels have good contact on the ground. When turning, more power is transfered to the outer wheel because in needs to travel more than the inner wheel.

Lock diffs have constant power on both wheels at all times even when one wheel is not touching the ground, required for crawlers.

Here is the Questions:

Is it good to lock the diffs on a Duratrax Delphi F1?
Is it good to lock the diffs on a Tamiya TT-01 and/or an AE TC3, both for drifting? For racing?

When purchasing a used RC, do you consider a locked diff as a custom mod?

Thanks in advance.