Can you just try it out temporarily how you're proposing without damaging your walls? I have my surrounds 4' above my listening position and my rears are right at ear level due to a number of reasons.
Dolby says
"Of course, it's not always possible to place your speakers exactly as shown in our diagrams. The diagrams give a range of placement angles, so you have some flexibility. Sometimes you'll have no choice but to mount the surrounds directly behind you, but if you follow the guidelines as closely as you can, you'll have good sound.
Ideally, your front speakers, high-frequency drivers, or tweeters should be positioned at ear level (when you're seated). Our recommended height for the surrounds is above ear level, as soundtracks are likely to be optimized for that location."
If you can try what you're proposing without destroying your walls, do it and see how you like it. Honestly with a lot of movies I have a hard time hearing if there's anything at all playing on the surrounds and rears vs. how good they sound due to placement
