I'll take another run at it...
Whether the rear sight of the HK is elevation adjustable or not, really isn't the issue as I see it.
Assuming the front sight only, post in a globe. If the post is centered in the globe, or whether the tip of the post is below or above the center of the circle circumscribed by the front globe sight, the post will subtend X minutes of angle from the theoretical center of the globe. This subtension, as measured in MOA on paper downrange will remain the same, no matter the distance, as it is a static relationship.
Thus if you zero your rifle at, say, 200 meters using the globe centered in the rear aperture, and set the target on the tip of the post, and you raise elevation your standard come-up for 300 meters, say 3.5 moa for a 300 meter zero, and maintain the same relationship between the front globe and rear aperture, and the target on the tip of the post, it will hit to zero.
The relation between the sights, as measured in moa of deflection, will remain the same, irrespective of distance.
Or did I miss something?