How much money do you want to throw at it. You are not starting out with a light frameset. It looks Identical to my TST and is the same size also. Mine weighed in at 1440g (3.18 lbs). So in order to save weight you will need some pretty light wheels and other components. I think you can get near 17 lbs for a reasonable cost but 15 lbs will mean you will need some weight weenie stuff, and that don't come cheap.

Mine is now just under 18.5 lbs, but has a lot of Ti parts besides the frame.