There is no way they are going to generate 850 chp on a stock vehicle. Even 750 is very optimistic.
BR-Performance are some of the best in the business in the BMW world. On a Euro spec M50i (with OPF sensors and secondary cats), they were able to get 680hp, and 750nM.
On my M50i (US spec), they were able to get 690 and 928nM. (Mine is the white one at the bottom of the page). US Spec vehicles only have cats in the DPs.
https://www.br-performance.be/en-be/...51-xdrive-m50i
The same company, tuned an X5M, and was able to get 700hp and 850nM on a Euro spec X5M. I "think" the extra 10hp over my car is because the X5M has a higher redline:
https://www.br-performance.be/en-be/...4-v8-bi-turbo/
These are aggressive tunes, with fuel injectors nearing their max flow capacity. With a US Spec X5M, you might get 710 chp... maybe 715 MAX.
Catless downpipes... aftermarket exhaust, intake upgrades? Sure.... definitely more. But in stock form..... I would be very skeptical of anyone claiming 750chp on a stock vehicle. 850chp?? No way.