I have attached a link to show the benefit of stainless steel over aluminum in the area of thermal conductivity.
http://engineersedge.com/properties_of_metals.htm.

Aluminum Thermal conductivity of 136 Btu / (hr-ft-°F)
Stainless Steel Thermal conductivity of 8.09 Btu / (hr-ft-°F)

What is the meaning behind this... Heat from the motor will be transfered through the aluminum intakes at a extremely higher rate than stainless steel (aluminum gets hotter a lot faster). The air entering the aluminum intakes will also heat up faster than in a stainless steel one.
Cooler Stainless Steel = Cooler Air = Better Fuel Mixture == Increased Performance.

Here is also an article from AEM's website stating this point.
http://trucks.aempower.com/ViewNews.aspx?NewsID=7

Transferring heat better is not always a good thing. Radiators = good. Intakes = bad.

If you need more proof of how stainless steel is better than aluminum we will be happy to provide it. There are many people who seem to have a false idea about stainless steel.

One last thought... Plastic? Why would you even think about it?