Ok i ran the fan today with the OS 18. stock header that came with kit no pipe got 24,400 rpm, added pipe (stock) got 25,500-25,600.
then open exhaust no header or pipe and got 26,117 rpm. So it looks like the stock header is really holding it back as we thought. i will clean the header up tonight to see if can get more out of it. i will also order a regular header for LA 15s and try some car ones. i think with some work should get 26,500-27,000 out of it. If still need more then can always jump up to a stronger 18.