Learn English – what is the diffrence between the best movies and the best movies

I prefer

This is the best performance I have ever seen.

over

This is the best performance I ever saw.

They mean the same thing (namely, that the person saying it witnessed the best performance thus far in their life) unless context implies otherwise.

This is the best performance I will ever see.

The sentence means they will never see a better performance; it's impossible for another performance taking place in the future to be better than this one.

It's probably most useful to think of holds true as a single phrase rather than considering holds as a modifier of true.

Holds true has the sense of is applicable. We take the an analysis of a past situation and find that it is applicable to the situation today, it holds true today.

Using is true doesn't work quite so well: it is not the correctness of the analysis itself that is interesting, it the use of the analysis in a different situation from that in which it was conceived. It is the combination of existing analysis and new situation we are considering. Holds true has this sense of applying something in a new context.