Probably it would work to some extent but today companies like Doyle invest big money in computerized study of sail performance and I am quite sure that a properly designed sail would be much more efficient. A flat cut geenaker as you are talking about is much a code 0 and that is a bit like a huge genoa.
It simply does not have a proper shape of a true downwind sail made to run.
this one is not flat like a code 0
In fact I think that a code 0 would make more sense if you only have a small head sail but if you have a big genoa (and a small head sail on a removable stay sail) than one of those will make a lot more sense with its added downwind capacity. Of course it will depends on the boat and how much sail the boat needs in light wind, I am thinking in terms of light boats with a very good performance in light winds.