A reasonably smooth belly pan was good for just over 2% on my Firefly, A-B tested multiple runs per set (I was genuinely disappointed. Sadface!)
http://ecomodder.com/forum/showthrea...ly-pan-79.html
But it's going to be different for every car, and I've seen ~4% published in case studies, like this one for an Audi A4:
The message spraypainted on your pan: thanks for the chuckle!