With that powder charge and a 250 grain bullet it should be moving at about 1602 fps. I ran a ballistic on a bullet similar, and it showed -2.3 inch drop at 125 yards if sighted in dead at 100 yard. It will be almost +2 high at 50 and +1.6 at 75 yards. So if you were to aim dead center in the kill zone of the animal you should be fine all the way around.

With that excellent quality bullet and speed and accuracy your obtaining is see no reason why the rifle would not be more then able to take a deer at those ranges with the right bullet placement.

As to how well it would do at 90 or even 100 grains of powder, all you can do is try the load. The Bighorn is a great rifle. I shoot a Wolverine and a Disc and they are very accurate with the barnes bullets.