my numbers are for a bone stock engine. We race an outlaw class here in cali every month with the GP spitfire and 25FX engines. all have to run a APC 9x6 prop. A few engines have turned up to 14,200 on a good day but most are in the 13,800 to 14,100 range. Now of course modified these can turn up to 17,000 on a 9x6.
Are you sure measured the head clearance correctly. .006 isnt enough at all. .016 is the lowest I have seen in this size of engine. .018-.020 is what I would think is ideal.