Im using a ER8EHIX. Except thats because the Koso head needs a CR/ER with its 8mm threads. (i think the first letters are the threads)
But anyway. Id go up to an 8, and even switch over to an iridium one. even if they dont work any better, that extra 7$ i spent makes me feel better.
Per my discussions with Brian, when I asked him about going to an 8, he suggested it himself.