I think the net gain with the 250 Gr is going to be long range accuracy, just due to the lighter bullet and high MV. The likelyhood of much getting up after being hit with that bullet is pretty slim.
As for the SD vs MV vs Expansion etc. I do not want to follow that in detail, as I have just followed a hugely long and detailed thread on another forum. Guys far more knowledgable than I hope to be with specific testing that had SD petty well down the list of contributors to terminal effects. IIRC the top honors went to bullet shape (nose profile), meplat size (not big or small, but just right 65%) and bullet construction, with velocity coming in close and SD well down the list.
As always placement is key. A buddy of mine recently killed a big Griz with a single 308 to the head - from a pistol (Vector V51). Griz dropped like a stone. So a well placed 250Gr 458 should do the job.