I am new to the forum and the audio world,but I have a different opinion on this. I too believed that Atmos was a gimmick and I used my Atmos Amp for over a year in a 7.1 config. Until I heard a demo of it for home theater. Really was amazed at the extra dimesion that was added. Its not the "greatest break through in 20 years" but I believe it adds to the immersion . Even good 5.1 mixes upscaled(Try Master and commander and U571) really do feel so much more immersive.
I do agree with the other guys posting that you rather spend more on quailty rather than adding more speakers, but I see your post didnt mention spending more or buying more speakers.
Your question of changing 7.1 to 5.1.2, I would say NO. Rather stick 7.1 and raise the rear speakers above the listening position maybe abt half a meter( depending how far your rear speakers are) and aim them down. I believe this will add a bit of a height feel for those Atmos tracks.