There is a large range in ORC certificates for 6mR's, going from 657 to 705 General Purpose Handicap.
This compares to Adam's 10's which seem to start at 630's and go down to 605's.
So Adam's 10 faster all day! 
data.orc.org/public/WPub.dll
Oops, just realised that link I posted before won't work unless you're registered with ORC.
It's free to register if anyone's interested in access to every ORC certificate...
The other issue, of course, is that there's a wide variation in the speed of 6 Metres due to their very different hull designs over time, so you'd need to specify how old the Six was. And for decent guidance you'd also need to give some information on what boats it's racing against.
Select to expand quoteSerb1980 said..
Adams, especially if you can modify the keel. that is a rocket
Yeah just don't try standing up in one. Not hard to be a rocket when it's just a shell for protected waters and mostly with just an outboard.
There is a really good picture of one kicking around somewhere from the 80's carrying a kite in a stiff breeze not broached but burying the bow with the whole rear half of the boat, rudder etc totally out of the water.
