Your thoughts seem fine to me. However I'd say the variation from 38 to 45 is a bit small to show much difference. Get some arrows a bit more
Extreme
and the effects may show up better...maybe try a 30lb spine and a 60 ! It will help to convince you which way you need to go.
Sometimes all the theory in the world is no help, but a quick test can show a lot.
Have you watched anyone else shoot it?
Good luck
Del