I've never had one, but I might need to experiment

Youd feel something, but not much. Like most stimulants, if the dose isnt high enough, it gets lost in your body - like a drop of ink in the sea.

Half energy for 2.5 hours?

3/4 energy for 3.5 hours

You'd most likely get half energy for 5 hours. The energy chemicals are designed to last for 5 hours, taking more increases the strength, not duration.

Half(more or less) the energy for the 5 hrs. Gosh, it says it on the freaking bottle