yes you can use it on k800i, for the w550 users you must flash the main with a fake main file so that far is able to boot in without haveing the phone to boot up
1. download fake main file
2. flash it in "MAIN" with far (only 6 bits)
3. Exit and start Far and it should go into in no problem without the trouble or bruteforcing on xs
4. exit far when done and go back into phone xs
5. find the MAIN file for your cell that you had originally before the fake main you loaded in there and JUST reflash your MAIN and just MAIN ONLY after that it should be good.