The tune alone should afford you below the 15's. Do you have much track experience with your sonic? Have you read all the advice in this thread?
IMO, the 1.4T manual Sonic is not the easiest to launch. Getting your 60' down to 2.3 or better will improve you runs greatly.
Surprised to hear you say that as I feel the sonic is about as easy as they come. Stand on the accelerator while at the line, let the ecu hold the revs, quickly/smoothly let out the clutch, and off you go (WOT the whole time). No drama, difficulties, or challenges during my few passes. Only thing that wasn't consist was my IAT's.
Admittedly my 60' times weren't all that impressive, but it wasn't trouble with launching as much as no boost off the line/low power in general. No issues with the launch, I hooked right out of the gate and was WOT the whole time. Has trifecta found a way to convince the ecu to hold boost at the line yet?
Of course I wasn't packing the kind of power you were... perhaps at your power levels launching was trickier?