He did try them - as someone wrote here, or at the similar thread.
The story is that he could tell - out of the two blacked-out frames, only after hitting two balls or so, which one was Puma and which one was Estusa, as they did definitely not feel the same.
I tend to believe that those frames he used, with the red butt cap were actually Puma Vilas (I remember that the Winner also had the red butt cap), Puma Super, from my memory, had the white butt cap, and perhaps the butt caps were "adjusted" during his years with the Super paint job to appease the sponsor and the more discerning public.
I will take a look at the Monte Carlo 1991 videos, and AO if I find time... I find it hard to believe that he won the AO 1991 (he really did hit the cover of the ball in that finas) with the racket he later decided not to play with.
I am not convinced at all that he ever played important matches with the Estusa frames (not to take anything from any of those rackets, since to me they are still better in feel than much of today's offerings), but the verdict is out, I wouldn't claim, based on the little we know, what the truth on the matter is.