A lightweight mixte with a racing or sport touring geometry and lightweight components (particularly a quality modern wheelset) will be a world of difference from your Superbe. Given your size a diamond frame will not only be hard to find but diamond frames of that size start running into design challenges as they are almost outside the range of what is appropriate when built up with 700c wheels. I don't think you'll be losing anything much by going with the mixte - a slight gain in weight perhaps, but not too much.