I am for a 24 second shot. But I have seen 2 time frames. One starts as soon as the lever is pulled/button is pushed. Those run 30-ish. The other starts as soon as the first drops of espresso come from the portafilter.
I haven't heard of anyone with a good 15 second shot. If they, they've either changed the technology of coffee extraction or they're brewing weak espresso. I doubt anyone could change the physicals of coffee extraction, but coming from high tech (designing circuit boards for supercomputers in the mid 80s) and seeing what you can get as a desktop computer these days, I have to allow that there can be technological advances that might not have seemed possible a few years before.
That said, Boyle's law is Boyle's law. Volume, pressure, temperature are what you have to work with. Unless your superheating steam (don't let your insurance company know), you're going to be just increasing the pressure. To not over extract, you need more coffee in the filter or you're going to under extract. Add more coffee, and you're getting good TDS (total dissolved solids) but I'm stuggling to see how you're going to develop the coffee through brewing. If I'm in your corner of the world, I'll just have to try a double shot straight up.