I'll let you know when I get mine, but they have been aluminum since '04 I beleive. Les is out of his alloy high flow impellers but will probably do another production run.
The impeller on my '07 is aluminum, but it is of the same design as the old phenolic impeller, that is to say it is not a high flow design. If you feel that you NEED a high flow impeller, I believe that Go Fasters bought out Les' inventory. Good Luck!