I don’t think it’s worth it T/TX owners to move to the 850P/B. There are a couple of draws (faster bus, supposedly a *slightly* better LED illumination, a little more battery life, small step up for Intel GMA). You lose the WWAN, though I don’t think that most people use that anyway.
With the TX750/770 having 100MHz more clock speed, even with the slightly faster bus speed of the TX850, the performance differences will be negligible.