I was just looking at this last night in the Summit racing catalog. Seems like a good idea to help the suspension follow the road better due to less unsprung weight. Overall weight loss of these springs will provide minimal MPG gain, but handling could be improved for maintaining more momentum through corners which would be probably the only MPG gain.
The ones Summit has supposedly have an adjustable spring rate - what a great idea! Tune 'em hard with adjustable shocks for the track days, and soften everything up for the commute. Espensiv tho.