The fundimental thing I recall that irrigation books would say about irrigating a slope was that you wanted to install the sprinkler head perpendicular to the ground. The placement of the heads had to then take into account the fact that the head wasn't going to throw as far on the uphill side as the down hill side.
As for knowing how much of a difference... well as a DIY, I just take a direct measurement myself. Basically, rig a few parts together so that you can run a single sprinkler head with a garden hose, set the water strength to match what I would expect once everything was installed, and then try a direct measurement on the hill you plan to irrigate.
If you reads Hunters guide to slop installs you do not want the head 90 degree to the slop.You will kill throw, as you noted and have dry spots.
Again, read the paper. I just did a small lot of 20K feet. Easy and head to head is perfect. I love #8 on PGP. Important thing I would say is head angle and shorter spacing on hills.
For his drop, I would be sure to use CV on the heads.