I went 13.5 one night on e.t. streets, only weight reduction was front swaybar.
But my car had all stock: (no porting) tb, maf, heads, cam, intk, headers, stock pulleys too.
only mods were 3.27 gears

accell coil

mac h-pipe and 3cbr flows.
with steeper gearing i think i could have gone 13.2-3.
I later went 13.2 @100 with just 1.7 rr's, and 1-5/8 unequal headers.
now i go 13.0 with h/c/i

but that would be my floating valves causing that.......
btw mustng90, 13.3 is damn good driving, for a full weight gt.