Learn English – The usage about “Never ever”

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.

Yes, this is correct. It means the last instance of the item it is describing, or, at least, that it is intended to be the last instance of that item. For instance, "this is the last ever monthly product analysis meeting" would mean that you wouldn't be holding any more product analysis meetings on a monthly basis - perhaps because product analysis meetings are no longer needed, or because you're changing the schedule to a weekly basis instead.