For what it’s worth, in February 2008, CFM clarified that its Leap56 predicted gains are compared to the latest CFM56 Tech Insertion configuration. CFM says it is targeting 10 – 15 percent lower specific fuel consumption; 15 percent lower maintenance costs; 25 percent longer initial on-wing life; a 10 – 15 decibel reduction in noise; and a 60 percent reduction in NOx emissions.

http://www.cfm56.com/index.php?level2=blog_viewpost&t=429

That would suffice to power a “next generation” single aisle.
I guess P&W will be sooner on the market with its GTF, while not sure if the GTF can bring similar gains then.
It will be interesting if any other engine company enter the market (Rolls Royce, MTU).