for example I wear a size 9 street shoe, but ski a 25 mondopoint boot which translates to a size 7.
That sounds like overkill to me. I go down one size. Two sizes and I would think the toes would no longer lie flat in the liner even in the skiing position. I wear a 9.5 street shoe. Standing up straight, my toes are pressed right up against the liner tightly in a 26.5 mondo with a minimal amount of toe wiggle; not what I'd call comfortable. In the skiing position, they back away from the liner a little and still fit tight, but are comfortable.
Then again, I have 100% completely flat feet. When I saw a podiatrist, he was shocked that my knees, hips and back don't constantly ache because of how flat my feet are. So, my foot is going to lie a little longer in the boot than the average person unless I put in foot beds with an aggressive arch.....something I've tried, but not enjoyed.