I have circled the hard drive screws in the picture above. In the older iterations of the Sonata, the smaller screws you are refering to could have been used. The is not the case with the Sonata III because the hard drive grommets were much thinner. In this case, the grommets are quote thick and on both sides of the tray as well.
Using the smaller screws is nearly impossible since they will not reach their respective holes on the hard drive.
As for Silverstone, I believe they considered adding a power supply option to their new Kublai cases but they threw out that idea before they were released. I think that Silverstone knows that whoever has the money to buy the majority of their cases should also have the money to buy a power supply.