Admittedly, I am very bad at envisioning a GG and how a certain focal length / comparison between two more FL's appears in practice.

I have a 450M - thinking of adding a second lens. I am unsure how drastic of a difference a 355mm would make vs the 450. The other option is a 10". Neither lens is close enough for me to actually mount and see the difference , which would ve the ultimate way of answering this easy question.