You could look at an ASA focal reducer corrector. They have a 0.73 reduction and correct for coma. So an F4 becomes about an F3.
http://www.astrosysteme.at/eng/correctors.html
Very expensive but are used in there fast Astrographs.
I wouldn't worry about there Astrographs unless you want to morgage your house but for around $2200 you can make a very good quality 10" f3.5 to f4 Astrograph yourself using readily available products and add the Korrector or there are a few Commercial F4 Newtonians floating about you could add the corrector to but the corrector would be worth more than the Commercial Newtonians. Check out my old Astrograph -sale pending
http://www.iceinspace.com.au/forum/s...ad.php?t=58061
Mark