I was sitting back hoping someone would answer. I suppose nobody is answering because there is no set answer. It all depends on the piece and the specifics of the geometry and ammo. If you can find an appropriate ballistics program you can estimate the difference using the same elevation setting.

I did a quick comparison using the apparent horizontal velocity in the last second of flight, and the vacuum range equations. At 5 degrees initial elevation, the range might be extended over 400 meters vs. a target 100 ft lower.

On the other hand for a target 100 ft. higher the range would be shortened by about 300 meters (and at 200 ft. difference the 5 degree shot is insufficient elevation.)

Range increase and decrease will not be the same because: 1. Gravity continues to accelerate/decelerate the projectile in the vertical axis. 2. Drag continues to slow the projectile's overall velocity--particularly in the horizontal. A target 100 feet lower will give some increase...but the next 100 feet of drop won't yield as much additional increment, because the projectile falls the next 100 feet more rapidly and travels downrange more slowly.

I'm no expert in these ballistics, I'm just applying basic physics and geometry, and recognizing that what I'm giving is only a very rough approximation. I would be interested in a detailed answer from someone with knowledge of real ballistic calcs.