It has a 63mm Secondary - which is 33% CO. The 8in. F/6 DOB version has a 50mm mirror.
If you run 'Newt software' it may help you determine if a larger secondary is feasable.
I had one for a year as sold by 'Astronz' in NZ - great 'light bucket' - but collimation had to be spot on. Coma noticable but expected in a 'fast' Newt.
The next GSO secondary mirror size up is 70mm which is in there 12 in.
http://www.dalekeller.net/atm/newton...t/newtsoft.htm
http://www.astronomy.co.nz/pub/GS600.asp