need help, fustrated

[95stangf]

when i first started modifing my mustang i added a kn filter and flowmaster mufflers and ran a 15.5 and 15.6 then i added 3:73 gears, h-pipe n/c, msd coil, 9mm wires,b&m shift kit and still ran the same then i added some slicks and ran worse 15.8. then i added c.a.i, 65 tb, headers , fpr, e-cam, pulleys, and several other cheap and free mods, with the slicks and still running in the 15.3-15.5 range i put the purple oil played with the timing up and down with diffrent fpr settings and still the same times. but when i use the nitrous with the right timing, tune up, right tire psi, full bottle it takes me to 13.7. my question is with out the n2o shouldnt i be running way lower than 15.3 on all motor with this mods?
i havn't done anything to my computer yet could this be the problem to take advantage of all the mods?
the motor has 118k.
any help will be appreciated