There will be a change in the audio “imaging”. User sensitivity to this varies. In vertical orientation each speaker is mono. A single horizontal speaker is stereo. In horizontal, stereo pair mode the “inside” tweeters operate at reduced level. I’m not sure exactly how a mixed orientation stereo pair will react.

In any case only you will know what sounds “best” (to you).

Once you trueplay I don't think it matters that much.

This would depend on the listener.

Everyone has their own idea of “best”. I know a fellow who doesn’t think his system sounds good unless SUB is in a corner and all available tone controls are set at max. That is not my idea of “best”, but he enjoys his system very much. Thankfully, I don’t often need to visit.

After flipping the right speaker on its side, I ran trueplay and found there to be a minor but noticeable difference between the output of the L / R speakers.  The vertical (L) seemed to “punch” through the mid to high notes more, with the now horizontal (R)  “softer”.  I guess this to be a function of how the tweeters are deployed based on orientation.  As the speakers are used in a large open concept space for overall ( as opposed to strategic) sound, I ended up adjusting the EQ balance to +2 for the horizontal speaker and am happy enough with the results.

And the human brain is also wonderful and does its own true play - after getting used to the sound in a few days of listening, you will find it quite natural. I might find living with the looks of this set up to be more disconcerting!