> Who knows how finely tuned these projects were or what their load was like before a rewrite. A rewrite in a new language gives you more benefits than just the new language , it also gives you the benefit of hindsight. The ability to fix mistakes made in the first implementation can go a long way.

That's a very good point with these rewrite stories - you could happen to rewrite some really poor code while porting and boom! Language Y is 50% more performant!