if you are very happy with the P8, but just want more travel, why would you get this BXT instead of, say, a P9? the P9 appears to have slightly more updated geometry than that BXT frame - steeper seat angle, slacker head angle, shorter stays. I'm not sure what the advantages of the BXT frame are.
if you are after a 150mm travel frame, have you also considered the FM08/FS713? While it doesn't have a threaded BB, that frame has more updated geometry. In particular, the stays are much shorter. The reach on it is longer too. After riding a few 29ers with stays in the 435-440mm range, i don't think i would even consider a frame with stays longer than 450mm, especially if i wanted to ride twisty singletrack on it.