Improve on your 60ft time and you will run a high 15.
This can be done through better tires, adjusting tire pressure for better traction, or just a better launch.. play around with rpms and clutch slip... That would be my suggestion for 15s with no further power parts... work on traction and get a 2.3 or better 60ft (not sure what Focus's can usually pull off anyways and never raced mine). You could also pull some weight out of your car (spare, jack, any junk in trunk), run it with a 1/4 or 1/8th tank (if not already done). I run my Mustang at around a 1/4 or just less.
Either way nice run!