The F-15s M61 cannon sprays 20mm shells at 4000-6000 rounds per minute and they travel at high velocity for miles. You'd have to be extremely careful about picking an unpopulated area to do this over. I doubt any air-to-air missiles would lock onto this thing (I'm certainly no expert). And as you pointed out, this thing is beyond an F-15s service ceiling, so it would be risky. A U2 or other recon aircraft could easily reach that altitude, but they have no weapons.

Maybe they're planning to get more aggressive/creative when it's over the Atlantic.