Kind of a tough nut on that one.
There is only about 1.5 grains difference in case capacity, with the .280 having the edge.
Short, fat cases are more efficient than long thin ones. For identical case capacites, the shorter case normally nets higher velocities, so your 1.5 grains capacity likely won't net an increase in velocity over the .284.
Add in the fact that with the .284, you would be able to seat the bullets out farther to allow more room in the case, you would likely have an actual capacity advantage , especially with something as long as the 180.
I think you're still looking at around 2600 - 2700 fps on the top end.