First off, there is more to a coating's quality than reflectivity.
Variations in coating's thickness and/or smoothness can adversely affect
the scope's performance. The thicker the aluminum layer, the more
reflective the surface will be. Many coating houses offer standard
(~88%) silvering as well as a 'super' (or whatever they choose to call
it) finish that it good for ~95%. This thicker coating is also
somewhat more succeptable to the aforementioned surface irregularities....
The 7% reflectivity gain you will get with a 'super' would be barely
visible, and then only IF you happned to have two otherwise identical
scopes available for back-to-back comparison. In the real world,
atmospheric conditions and other variables vastly outweigh the slight
brightness increase a super-dooper mirror will fetch.
The mirror's shape is at least as important than the coating. Are you a
member of a club? If so ask for help (or google the simple proceedure)
in performing a 'star test' on your scope. With any luck, someone in
the club will also have a knife edge or focault tester laying about and
will be willing to do a more comprehensive surface test. If your mirror
has problems, there are several businesses that will re-figure it for
you. (Many of them will do a cheap/free figure test.)
If you decide to go ahead with either/both services, I can personally
reccomend Steve Swayze and Spectrum Coatings (I got their 'enhanced'
aluminizing.). My old 10" Blue-Tube Coulter was transformed by a
refigure and re-silvering. (Replacing the Flintstonian secondary 'beam'
with a real spider, and other tweaks didn't hurt either! My scope is
still butt-ugly, but it works VERY nicely - for a puny 10" )
If the old surface is knackered, 80. If in near-new condition, 10~25,
depending on the surface quality of the OEM aluminum.
Unless your current coating is degrading, or the optics need help, I'd
save the $100+ for a new eyepiece or a mounting upgrade. Also, poor
seeing and light polution will degrade any scope. If you don't use it
at a good observing site, your $$$ will be largely wasted.
Even with clean air and darks skies, a 30$ cooling fan installation will
do far more for your viewing.... http://www.fpi-protostar.com/bgreer/