the FMF's are infamous for this leak at the header, best fix ive found to assemble the pipe onto the header with gasket sealer, i have had success with this but it needs to be redone every now and then.
The ultimate solution i am getting around to is to have a new flange machined to fit precisely inside the pipe and thus prevent the leak.
Another more hospital job is to split the pipe about and inch with a hacksaw in 4 places down from the open end that slips over the flange and put a good quality exhaust clamp over it to squeeze it together, this however means defacing your header, its a last resort for me.
the leak is not a major problem, just a messy one.