Those are respectable times (the first time I drag raced my 93 5.0 I ran 15.8 and improved it during the night to 15.3 but that was all that day)... but I understand your disapointment... as stock ST are having mid-high 15's and with the mods you wanted to go even faster!!!! We all do :)
But don't worry - there's so many variables... I bet you were having hocking up problems... but there's also the temperature, the stickyness of the track, the time of day (the evening get's cooler, but the air density also increases), the weight of the car (fuel/car seats :).
When I first raced my stock ST (I also had a car seat and full fuel tank - last minute racing outing:).
The whole night I was spinning all the way from 1st- 2nd gear... and no matter how much I changed it... I would either almost bog it down or spin... My best that day was 16.1... I was disapointed I didn't make it to the 15's... but l8r when I checked my slips... I noticed I should've had a much better time, as my other slips even w/ a slower final ET had a lot more top end speed/times.
There's always next time and better conditions!
Here's some clips of my night