I believe a lot of the top pianists take their own pianos with them. I'm pretty sure that Schiff's marathons at the Proms when he played all of Bach's preludes & fugues were on his own personal Steinway D274* (274 being the length in cms).
I have read that Horowitz, who many regard as the greatest pianist of all time, used his own piano and he had the keys set very "light".
https://www.youtube.com/watch?v=FxhbAGwEYGQIt's noticeable that he has his fingers very straight when he plays. If he were my pupil I'd soon sort that out!

That's the Goldensaal in Vienna. Jan and I attended a concert there in 2018 and had seats right at the front of the balcony. The Bösendorfer showrooms are in the same building so he could well be playing one.
Wikiepedia makes no mention of his piano's keys being underweighted, but...
Horowitz's hand position was unusual in that the palm was often below the level of the key surface. He frequently played chords with straight fingers, and the little finger of his right hand was often curled up until it needed to play a note; to Harold C. Schonberg, "it was like a strike of a cobra."[1] For all the excitement of his playing, Horowitz rarely raised his hands higher than the piano's fallboard. One of his students, Byron Janis, said that he had tried to teach him that technique but it didn't work for him.[25] His body was immobile, and his face seldom reflected anything other than intense concentration.
Maybe the stuff about underweighted keys is hearsay. Marcus Roberts, the Oxford dealer I bought my piano from, has numerous videos showing that the weight necessary to move the key should be about 50g.
*Having said that, I am sure I've seen a video of Schiff extolling the virtues of a Bösendorfer. Although now a naturalised British citizen living in London, Schiff is from Hungary so Bösendorfer would probably be the top manufacturer anywhere near him.
Edit: wow - just look at that... I want one!
https://www.boesendorfer.com/en/news/Andras-Schiff-plays-at-Konzerthaushttps://www.youtube.com/watch?v=LvekkZnE8BU