The other thing those HTA68 vs. 18g graphs demonstrate is the really minor effect that the billet wheel really has.
non-billet 18g is essentially identical peak HP, with the billet HTA68 showing a minor 3% average torque advantage... However, that 3% gain is less than the difference between the worst and best run on the three dynos of the 18g. Meaning the advantage is 100% in the noise.
In other words, a non-billet wheel will perform, for all practical purposes, identically if it's sized comparably. So bargain hunters, watch the classifieds as those who don't understand physics dump their 'old tech' turbos in favor of the new hot thing that will net them a virtually unmeasurable gain. I already picked up an 18g for CHEAP, cheap enough that even if it needs rebuilt, I'll come out ahead.