Here is Hinson's website and you'll notice that the Banshee system is aluminum. It might be just me, but Hinson has a lot of experience in this field and I'll trust what they are saying.
http://hinsonracing.com/product/complete-billetproof-conventional-clutch-kit-yamaha-yfz350-banshee-8-plate-1987-2006/
Most all the factory teams in MX and SX use Hinson and swear by them.