transparency without transmission

[chris.holland7P2HP]

I'm rendering a building as a hero; I need some close-up shots. It's low-light. Transmission samples are killing me. I need a transparent shader with reflections to see through the windows without any form on transmission samples required whilst still keeping a reflection of my HDRI map.

I'm using C4D if that's any help.

Thank you!

[Stephen.Blair]

The only other "transparency" is opacity, but then you'd lose the reflections.