In Ralph's survey of Strauss's Also sprach he mentions Lorin Maazel only recorded the work twice - in Vienna and Munich. In fact, he also did so in London with the Philharmonia Orchestra. These sessions were done at Kingsway Hall in June and August 1962 (he also recorded Till Eulenspiegel at the same time). It was released on EMI/Columbia in 1963, so long predates his later ones.

I am also certain that these Strauss recordings were the first time Maazel made a studio recording with the Philharmonia.

One reason they might have been missed is because they have not had a CD issue - at least outside Japan, and even that is out of print. I have the CD somewhere and don't remember it being that bad; on the other hand, I don't remember it being strikingly good either!

Also sprach was, as it happens, the last work that Maazel conducted with the Philharmonia (with Ein Heldenleben) shortly before his death. A radio broadcast exists of it - if you think Sinopoli is slow then do try and listen to that one because I seem to remember Maazel took the work close to 40-minutes.

And yes, I agree with you about Sinopoli/NYPO. Like all of his New York recordings it is fascinating.