You don’t lose much at all from quality gold-over-silicon mirrors. American Photonics claims 99.6%. That’s so trivial, don’t worry about it.
Molybdenum mirrors are listed as 98%. However, they are nearly indestructible due to high thermal conductivity. I like to use molybdenum on the #3 mirror, the one most likely to get dirty from smoke, at our makerspace because this machine is 225W and a dirty silicon mirror will get so hot it bubbles off the gold plating. And, being a makerspace, most users are casual and don’t inspect and clean so this does happen.
Molybdenum mirrors get hot in operation due to the losses, but they can absorb a lot of wattage without overheating the front face. It’s literally a polished, uncoated face of a solid molybdenum puck, and molybdenum doesn’t oxidize.
I did have some lenses explode from the power density. Like the beam bored a hole through the zinc selenide and the user reports it was cutting poorly… like, yeah, no lens. The #3 mirror was moly but had such a baked-on coating of brown crud I couldn’t clean it. But, the part of the surface I could get the brown crud to come off of appeared undamaged.