I thought DJ that you would have realised its not all about overall weight.
Every gramme saved off the weight of the wheel rim is worth 5 saved off the hub.
I use a set of Dymag carbons on the track and the difference between those and my magnesuim rims is considerable, easily worth £900.
IMO Light weight rims are the biggest single improvement you can make to a 996/998 in both performance and handling terms.