It depend's on how in-tune with your car you are.
Some people claim they can feel a difference with an additional 5 hp.
Most will say that around 20 to 25 will make a noticeable kick.
Same with ET's.
A tenth, can't really tell.
3-4 tenth's, you will know.
You will feel torque (acceleration). You will probably
not feel HP especially if it's an airflow mod that only
adds anything up top; all that will do is maintain the
torque level further out in RPM. Can you feel a "not
falls off quite as much from 5900 to 6000"? Maybe
if you shave the hair off your butt-o-meter and sit
naked in the seat.
Some people from the placebo group always respond
in medical studies. This effect is what keeps the low
end of the automotive aftermarket thriving.
ive heard 10 hp (+/-) equals roughly a .1 second qucker et (+/-) so if you can notice a .1 second decrease over a 12.5-13.5 second time period, your a better man than me. my car needs about a 25-30 hp increase i would say for me to go: "whoa this things got more grunt".