It's the design of the wheel that makes the biggest difference. Even Garrett has new design billet compressor wheels now, the billet GTX GT42Rs flow way more air than the older ones, with essentially the same size wheel. Cast vs CNC machined or heavy vs light does not make a compressor wheel flow more air.
From that article "For either of these turbos, a cast aluminum wheel using the same dimensions and blade designs would have performed equivalently in most aftermarket applications. "
http://www.turbobygarrett.com/turbob...ochargers.html
"Forged, fully machined wheels (billet) for expedited release"