The distance does not make a difference when comparing velocity achieved by 2 powders. The one that produces the highest velocty at the muzzle will also be faster at 250 yds, all other things being equal.

If BH 209 functions well in your gun, is accurate (or at least as accurate as T7), AND produces significantly higher velocity, then it makes sense to use it. What is a significant amount? That's for you to decide, but even a 100 fps difference will help noticeably at 250 yds. From what I've seen, 100 gr or more of BH 209 really starts to work well with bullets on the heavier side. If you're leaning toward a bullet say around 300 gr. or more, my guess is BH209 will be the way to go. May not make so much difference with a bullet around 200 gr.

I like to sight in so my bullet is no more than 3" high or low of the point of aim, out ot the max distance I intend to shoot. I don't beleive T7 or BH 209 will acheive that with a MPBR of 250. For reference, that about what you get with a 30/06. Not saying hitting reliably at 250 is not doable.