In my personal experiences i always take a taste from the bottom of the plant and in fact the potency of a bud at six weeks just might be higher than that of one that is eight weeks so why not give it a try and let us know.
Its ok for the fan leaves to turn yellow. Thats just the plant soakin all the nitrogen it can get. Its perfectly normal and actually gives the plant better flavor when starved of nitrogen at the end of harvest.if the top colas are ready i personally would harvest the whole plant, but if you are...