YEAH ive heard that before...
thats at 10psi....think thats to much for that guy? would the 66 be better maybe...probly to big for that little motor though..
Talk to a guy called "mnflats" on dootalk, he ran 11lbs on his 53 in his 600sdi and it made it 1 ride........then the turbo blew. A rebuild is about $1000 and no go until the turbo is back in. You might get away with 10lbs, the newer 53's are designed better, but probably only good til 8lbs on an older one.
It will still be a good sled with 6-9 lbs, but trying to get too much out of that little motor will only cause more problems IMO. Not sure about the 66, you'd have to talk Aero and see what they say.
Good luck man! Hope you get it figured!!