You could put the step in the room, as you drew, but you are limited to an 8.5" high riser, without a step by building codes.
Now that may or may not work, with regards to the second row being able to see the bottom of the screen. For that, you could draw the room out to scale, from a side view perspective.
If you have the seating, then you can measure exactly where eyes are, and then put heads in the front row and see what height the screens needs to be at. Draw in your projector, with it's
throw distance, and you also see if there's any riser clearance issues. With all that, you are not guessing if it all works together.
Why will you need acoustical treatment on the riser ceiling? You usually treat the first reflection point for the front row. I would suggest you don't overly deaden the room with excessive acoustical
Why give up the extra width and volume? As Big mentions, getting the side surrounds further off the seating is a nice plus. I expected you would be in an underpinning/benching situation at this point.
Hence I didn't really touch on that side wall previously, because that area could have been used for benching, and a 'stage" be the front bench. What I suggested was more about being able to take the
riser full width, while bringing those steps outside the room, for safety. (The two boxes are indeed wide steps, and all about the nice luxourious looks of Big's "BigA$$" steps. They are also about meeting code
and having the riser tall enough to enable proper screen sightlines.)
So what exactly is outside the theater entry? I myself like the idea of a feature wall, and some inexpensive detailing right outside the room. In my case, it is about being able to introduce some wow factor
for my next room, which is to be a black cave.