i've actually gone up to 375x2 for stiff legged, without any back pain whatsoever. i've heard that the lower and middle back are the only parts of the back that need to remain straight. if you've ever seen konstantin konstantinovs deadlift you'd find that his upper back is always rounded even when he's doing heavy deadlifts. (KK actually was born with bad spine).
it works for me but as you said it can be considered an advanced movement and everyone's body works differently.