My first run not modded was around 15 something
Got it down to 14.98 in the same session
The time I went with exhaust, pod filter and stock boost did high 14's couple of low 15's
Last time best was 14.18 with bald tyres, it will be in the 13's next time
If a stock skyline can't break 15.5 it would be because of the driver .... not the car