And they do pretty darn good compared to a lot of GM's aluminum offerings, too. They flow nearly as well as a set of LT1 heads (which, of course, don't work on a GenI block) - because the head design was taken right from the LT1. They even do better than the L98 Corvette head!
http://www.corvettefaq.com/c3/goodwrench/gm3504.htm is the Vortec portion of a buildup of the el cheapo GM Goodwrench 350...