I don't think it would be a problem to have the center surround somewhat higher than the side surrounds. I ran some surrounds that way for several months.

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gonk -- Saloon Links | Pre/Pro Comparison Chart | 950 Review

My suggestion would be to leave your surrounds at 6ft high,then just install your rear surrond center at 7.5ft;just make sure you calibrate all your speakers with your meter at the same level,forget about THX this 6th speaker makes all the difference in the world.Better sound field,better wrap around and sounds come from all around you(on the movie Sream 3 i hear voices and sounds all around,a very real surround sound,rear left,right,back...awsome)

Thanks, Gonk and Eurorom, for your thoughts
on my surround speaker placement...I appreciate your time!