HP increase: On paper vs. in the Seat

[geno]

I often read claims about increasing HP by 5 or 10 by changing out one thing or another.

I have a '99 MY Boxster and I seriously don't know if I would even notice a 5 HP increase (for example). That's only about 2%.

Is there a point (or percentage increase) where it would be easy to say: "Geno, you're definitely gonna feel the difference!"

I appreciate your thoughts.

-gene