Here is some information on Messier's telescopes.
http://seds.org/messier/xtra/history/m-scopes.html
Speculum has a reflectivity of about 63% after 6 months in a damp atmosphere.
http://www.iop.org/EJ/abstract/0950-7671/24/9/308
Aluminium has a reflectivity of 87%. That means Messier's speculum 8" is equivalent to a 5.8" aluminium mirrored Newtonian.
(8 squared x 0.63 squared = 5.8 squared x 0.87 squared)