I think there are some misconceptions about the wheels out there.... yes there is a bit of 'bling' factor that people are willing to pay for, and perhaps 'Billet' is equal to Titanium as a buzz word to sell by. Who knows.
I would agree that differences in comparing a cast vs billet wheel OF THE SAME DESIGN, would show similar results on the dyno, but keep in mind that most small run, and test compressor wheels are made by CNC. Generally these are of a newer design and potentially better performance than standard cast ones that have been around for a while.
But, to stir the pot
, what about the material differences? A cast alum wheel is weaker than a billet version even if it's the same alloy. Most good billet pieces will be forged and therefore stronger. What does that mean? From some research I have done over the years, a stronger wheel will have less blade deflection, you would be surprised at how much the fins bend at full load, and you will have a performance gain from the added stiffness. What people should be talking about is the same wheel made of Titanium and see what happens.... ;P
$.02
-Bob