Have you considered the used market for a frame? Plenty of high end light steel frames from back in the day available. I will agree with many of the others to say that light isn't everything. I've got 25+ steel bikes and lightest is not always the best riding, the fastest, or even the best climber. It's always a compilation of things.
As an example though, this one was picked up and built back up last years. Mid 90's Giordana Excell Podium tubed frame. I've got a couple of bikes made with this old tubeset and it was quite light. This bike built up exactly as in the picture below is 17 lbs 14 ozs. Not bad at all for old steel.
Pretty much excellent frame except for one mark in the paint on the top tube.
So, just throwing out the used idea as that opens up a whole new world of quality bikes for what can be a great price.
This early 90's bike was picked up in mint condition. Threw on some modern wheels and set a
PR for my 25 mile route on it that I can't seem to beat with any other bike I have.
Columbus TSX tubed, carry's speed so well.