Disclaimer: I have not owned an 800, S or otherwise.
...However, our regular riding buddies include an 800 LE, a 900, and a quad rider on an 850 Sportsman. So I've gotten to see first hand the differences in on-the-ground performance.
Top speed
The 800 LE and the 570 are absolutely, positively dead even. WFO on the Black Rock Desert playa (the flattest place on earth) with the throttles pegged, they stay dead even. After 5 miles at top speed, they will still be within 10' of each other.
Acceleration
The 570 seems accelerate just a little bit faster than the 800. It's enough for bragging rights but not any practical difference in the real world. The 900 (and the Sportsman) both kick us all to the curb, anyway, so that keeps us humble.
Hill Climbing
We don't have any huge sand dunes we ride regularly for hill climbing. What we
do have are long, rocky, steep hills that are part of trail systems. Here, our 570's transmission shines - it regularly scampers up hills the 800 cannot. It would not be out of place to say the 900 "keeps up" with the 570 on ascents of this type by virtue of horsepower.
Technical, tight terrain
Although the 800 is also a 50" machine, the 570 seems to make easier progress through tight, turn-filled bumpy trails. Not sure why, I would point to the transmission again? Still, there it is.
Last weekend, we were delighting in riding on single-track. It was great to be able to go somewhere that was not normal for a side-by-side. The 900 is too wide for this kind of trail and would regularly have to "go around" and meet us. The 800 in a 50" size handles this, too, as does the Sportsman (of course).
Since we routinely ride tight trail, and ride with ATV's, we love our 570. It has gone everywhere we've asked of it, and is better than our driving skillz.
On the other hand, if we rode wide groomed trails or dunes, of course we would have a different machine. <shrugs>
In our environment, the 570 has proven itself to be better than the 800 in every way except one: the name "570" sounds so much smaller than "800".
Flame away. It won't change the facts any and won't hurt my feelings a bit.
All the best, James