There is absolutely nothing wrong with this recording. Mattila can sometimes be a problem. She tends to go a bit off pitch and can become a little squally on top. That is not in evidence here. Perhaps the engineers helped her a bit. Perhaps she was just in very good voice. I don't know.
Heppner is, well Heppner. And that's pretty great, in my book. Clearly of the Melchior school, and much better than in the disappointing (at least to me) Met Tristan telecast.
Solti, I think, was more exciting (which is what you go to Solti for, after all) in the earlier recording.
Now, here's the deal. I like the Sawallisch recording better. He is more from the "kappelmeister" school, and less of the "Übermäßiger Stern", if that means anything to you. Heppner is a little freer of spirit, if that makes any sense. And Studer is more interesting than Mattila. I also prefer Weikl here to van Dam. I simply feel its a little late in the game for van Dam. Of course, that's a personal opinion. Isn't that what all music is?
As for the Karajan recordings. I am awed by Karajan's talent. I love his Ring Cycle (except Rheingold). But I think he over refines "Meistersinger" and the undervoiced, overmiked and way over-recorded Kollo is always, at least for me, objectionable. Heppner is much to be preferred, in either recording.
And perhaps one of the recording companies will discover the fabulous American, ROBERT DEAN SMITH, put him in front of a microphone, give him a great conductor and allow him to put to disc the greatest Walther (and Lohengrin and Parsifal and Siegmund) of this generation. Hopefully, soon.