Visualize the output of a Trowbridge-Reitz Half Vector Sampling Function
So for anyone who was interested - after studying the source material by Walter, I became convinced the error had to be in the rotation of the slopes back into "normal shading space".  PBRT does this in a one-liner: "CosPhi(wiStretched) * slope_x - SinPhi(wiStretched) * slope_y;" etc etc.  I had split it out into separate sinTheta, sinPhi, sinPhiX steps.  One of those steps wasn't correct so the rotation wasn't unstreching correctly.  

So the correct visualization for a GGX normal distribution of half vectors is this: