The average water buff bull probably weighs better than 800 kilos ,so yes more than 1500 lbs.
I haven't shot a buff with the 303 yet but it will happen one day!
I've been with a hunter that shot some with a 303 and they were all one shot kills.
I've never had a problem putting down camel,scrub bulls,donkey,horse,deer ect with my 303 using 215gn Woodleighs.
Good luck,
Al