OP: Has more to do with getting the proper dispersion and listening angle. :)
Do a little research into the angle of listening (dispersion) vs. the size of the moving diaphragm and that will answer a lot of your questions and explain my recommendations.
Of course, nothing wrong with you learning by doing! :D
Should also consider, if ESL’s worked horizontally, why aren’t they all horizontal? :) The answer is, again, the dimensions of the diaphragm.
If you have 2 ESL's already, and want to experiment, go ahead and turn them 90 degrees and tell us what you find.