You do understand, don't you, that High Noon's "Alter Ego" holster is made to a different standard than is the Galco?
That is, a large part of the extra thickness you noted is caused by the leather choice High Noon made.
The reason the Galco "Tuck-N-Go" works so well is because the pouch is made of thinner, more malleable material than High Noon's is. That choice is due to Galco's designers' genius, and their corporate ability to "think outside of the box."
High Noon chose to use wet-moldable leather for the "Alter Ego"'s pouch because that is the "gold standard" for making holsters; however, it is not always the best choice for every design.
I suggest that it is dangerous to carry a loaded defensive weapon off-body, for instance in Galco's "Day Planner." Off your body it becomes subject to a snatch, or to your own forgetfulness ("now where did I put that..."), or to the uncontrolled inquisitiveness of a co-worker or a child.
Before you get yourself too deeply into holster critique, please let us know about your experience.
For how long have you carried a concealed weapon?
Have you ever done any leather work?
What shooting training have you had?