If the object is purely to improve one's sight-playing abilities at the piano, there would at first glance appear to be little useful purpose to be served by trying to play at sight music which is too far beyond the reader's technique to play at all (that's to say even after practising it); there might be other useful purposes in so doing, however - i.e. discovering more about how the music is made, both musically and pianistically - and the exercise can sometimes have the additional pay-off of inderctly improving one's playing techniques in any case. I know this for a fact - at least in terms of my own experience years ago - for, although I am not a pianist per se, the act of trawling through sheaves of Liszt, Alkan and Godowsky certainly helped me to come to terms with writing for the piano, even if it did little to improve my sight-playing, let alone turn me into any kind of pianist!
To return to the question proper, I would say that, to improve sight-playing facility, trying, for example, the "48", Beethoven sonatas, Chopin's, Liszt's, Alkan's and Godowsky's Études, the Brahms Concerti, Carter's Sonata and Night Fantasies, Prokofiev, Messiaen and Xenakis - to the extent that any of the works concerned are not unduly beyond the player's technique to get the fingers around in the first place - would be good ways to start; that said, simple score-reading exercises are also indispensible.
Best,
Alistair