Just an update, in case anyone is considering this tank and curious... I filled the tank all the way to the top on Sunday and watched him for hours. It did not take him very long to figure out where the opening is and has shown no signs of stress. He even started making a little bubble nest yesterday but I had to interrupt him as I'm doing daily water changes due to high nitrates (it's a new tank setup and my plants are not yet fully established) and had a limited window of time during which to test the water and do the next change.
So far I'm very pleased and it's amazing to have another angle from which to view the tank! Sometimes he'll go to the top and have to "recall" where the opening is. He never seems stressed and acts no differently nor any more "confused" then when he "bumps" into the sides of the tank. It's equally "unnatural" to have a clear wall on the sides as it is on the top, or so it seems.
One note for someone considering purchasing this tank but not filling it to the top... my experience, in the week prior to filling it all the way, there was a HUGE amount of condensation on the top. It actually looked pretty cool but it made for ZERO visibility from the top. It wasn't even a bit distorted like looking down into a more traditional setup. These were large drops of water stuck to the top... up to the size of a dime and covering the inside of the top so there was, basically, no visibility whatsoever.
The tank, however, is a joy to look at and Pesky seems just as happy as prior to filling it all the way to the top. I very much anticipate getting a second tank (maybe the 12 gallon version) after my work travel settles down a bit.