If all that mattered was yield, we'd all grow photo periods because, even with the currently available good yielding autos, photo periods are still superior in that department. And F1 cross is a collection of all the available genetics of the 2 plants, and all dominant genes prevail. You can't select for the traits you actually desire till later generations.
A super auto / semi auto will never be a stable strain because once you go to F2, the genetics begin to shake out into photos and autos again, thats how you find the autos in an auto/phot cross. The you take those that auto and the earliest males and make F3 auto seeds. These will be a larger percentage of auto flowering plants F5 is considered the beginning point of stabilization.
Creating an F1 strain is not work, it is only step one in a long process.