Gershwin Half-speed Mastering

One of my favorite LP's is "Gershwin on Broadway" with Michael Tilson-Thomas and the Buffalo Philharmonic. I bought it in a used record store in Greenwich Village for $2.98 over 30 years ago.
It turned out to be one of the best sounding analog recordings I own: broad sound-stage, precise imaging, dynamic range etc.
  And the performances of Gershwin Overtures are also outstanding.
It was used as a model for the music in the movie "Annie Hall."

The record  is still in excellent condition and the sound is still wonderful.
I liked it so much that I bought a half-speed mastering of it, that I knew to exist, on eBay, thinking that it might even sound better.
What a shock!  The sound was now homogenized lacking all the space and imaging that the original possessed. It almost sounded digitized, sharp and clear with no texture.

Now that I think of it, half-speed masterings were never an improvement on high-end sets. They were made to sound better on the majority of mid-fi sets.
Has anyone else had this experience?
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I'd be remiss if I didn't comment more about the music in a music forum.
This is literally one of the best Gershwin discs I own. The performance by Tilson-Thomas and the Buffalo Phil. Is scintillating.  Truly idiomatic.
And the arrangements of the overtures by someone named Pearson, are masterful.
Its a record worth looking for in sound and performance (but not in the half-speed mastering.)