Is a lighter steel frame really going to make much difference? You might save a bit with a lighter steel frame but not a lot. Most of the weight in a bike is the combined weight of the components. e.g. my bike weighs 10.5 kg however the frame only weighs 1.8 kg.
Bite the bullet and buy a titanium frame. Yeah they are quite a bit more expensive however they are extremely strong for their weight and very comfortable to ride. If a lighter frame is really the solution, a titanium frame will be better value in the long run. Much lighter than steel, strong and comfortable like steel and many companies offer real lifetime warranties.
DJ