1O" and under scopes for planetary are ok if you don't mind black and white views of the planets at higher magnifications ( as the image is too dim) to excite colour vision.
There is simply no subtitute for aperture and light gathering. A 12" F5 will be a far superior planatary instrument than an 8" F8. Long focal length instruments just had better reputations 30 years ago because they had so little departure from a sphere and there was less skill about to make faster mirrors for the amateur. A slight difference in central obstruction between different F ratios is nothing compared to the importance of smooth optical surface.
As far as I know theres a S/H 12" F6 conical mirror available here
http://www.iceinspace.com.au/forum/s...ad.php?t=31474
Hope this helps.