Im pretty sure it affects duration by an amount too
If it does it would be very, very ...VERY easy to check.
takes 3 steps to truly judge.
1) open your buff bar.
2) drink an agility pot with no EP, see how long it lasts.
3) Drink a strenght pot with EP, see how long it lasts.
The buff bar gives you exact numbers, so if you wana know...go and check.