I use a 8:1 rear sheeting and spend a lot of time in the gym. I find myself wanting 9 or 10 to 1 when the wind picks up. So, if you can use a 6:1 center, you have my respect.
I used to think I was sheeting hard until a friend sailed behind me as a coach. His comment was " I know it feels good cruising around with a full sail, but for racing you really need to sheet hard and flatten that sail out".
I sheet a lot harder now.
I liked the center sheeting but there was just too much friction to overcome when sheeting really hard.
On the Marstrom's the rachet block for the center sheeting is on the front crossbeam where you have ~160-170 degrees of wrap. It seems to work ok.