"I never heard his M5 (mono) and have wondered about it"
Very fast throughout. I've never found it to be very interesting. It is well played, but so too is the Scherchen/Philadelphia M5 on Tahra, which is vastly more fascinating from an "interpretive" (i.e. nutzoid) standpoint. Scherchen is just insanely fast during the faster portions of the second movement, yet "Philly" keep up with him perfectly. Then he takes nearly 15 minutes on the Adagietto. But with Philly's incredible string section, that simply isn't a problem. It truly is a realization of the nutty stuff he was trying to pull off in Toronto and Vienna. In Philadelphia, he had an orchestra that could actually pull it off.
Personally, I think Walter's best Mahler recording is the N.Y./Columbia Records "Das Lied von der Erde".