And a quick bit of googling says speculum max reflectivity at 6500 angstroms of about 75%.
A modern aluminum mirror does about 90% . . . but I am not going to do the arithmetic on figuring out angstroms to nm . . .
So I'm going to guess about 20-30% less reflectivity. Not bad.
I'm also going to guess uncoated eyepieces. . . even still, I have to think that the cutting edge back in the day can figure speculum beyond 1/4 wave, and have > 50% ultimate transmission to eyepiece.
So the 6" would behave as a modern ~4.5"
That data was for modern electrodeposited speculum with 45% tin.
"The reflectivities of electrodeposited speculum (a copper-tin alloy containing preferably 45% of tin) and of speculum prepared by evaporation in vacuo have been determined over the range 4500-6500 A. The reflectivity of the freshly polished electrodeposited alloy containing 45% of tin, which is the ideal composition from the viewpoint of maximum resistance to tarnishing, varies from 63% at 4500 A. to 75% at 6500 A."
According to Wikipedia, the common alloy in use had a tin content of about 31%.
1 nm = 10 angstroms. Transmission = 63% in the blue, 75% in the deep red.
Visually, deep red is of little use and even modern coatings it lost 10% in 6 months.
I'd figure something closer to 60% at best based on polishing rather than electrodepositing, material differences, tarnishing. That would be 45% transmission compared to two 90% mirrors.
But this thread is about the figure.. testing a 180 year old mirror would require polishing and that would require refiguring.