No offense but many companies are not truly expert. There is little physical size difference between a 54 inch tall 2.35 and 2.4. You are widening things only 2.7 inches or 1.35 on each side. The problem is that you would like the image to be black masked by the screen frame or variable screen masking. Lets take a 16x9 image on either a 2.35 or 2.4. Unless you add drpo down masking on each side, you will see light screen material on either side. No black bars. So adding masking to mask a 16 x 9 is nice if affordable. If you have a 2.35, most every wide screen image will completely fill the screen letting the screen frame mask the side edges. If your screen aspect goes to 2.4, many wide screen movies won't completel;y fill the screen. The side black edges of the screen won't make the edges sharp because there will be a small amount of screen material unlit.
A 2.4 screen is ideal if you have variable screen masking that moves horizontally from the sides allowing you to in essence make that 2.4 screen 2.38 or 2.35 to remove any unlit screen. It can move all the way down to 16 x 9. Everything will be perfectly masked. BUT SUCH SCREENS ARE VERY EXPENSIVE. When one has just up down masking to make a wide screen 16 x9, the right way to go is to get a 2.35 screen and just overscan a bit, filling the entire screen, edge to edge, You lose a little bit on the edge but your masking and its effect on image quality will be the best.
Your eyes in a dark room adapt to the brightness or lack of it. In CRT FP days, we lived with low foot lamberts. Things were fine. We quibble about what is ideal now. 12 ft lamberts or so. 10 ft lamberts isn't bad. It doesn't look 17% less bright, just a tad dimmer to your eyes.