Armor mode works likes this:
It absorbs 35% of the dmg done to you. If I hit you for 100 points, you lose 65 health and 35 energy. If you only have 10 energy and I hit you for 100 points, you lose 90 health and your last 10 energy. So yes, coming out of stealth mode with 10 energy and going in to armor mode is pretty much useless.
This isn't my own work..... This site is extremely helpful :
http://denkirson.xanga.com/742784256/crysis-2/
As far as stealth mode amplifying dmg.... I'm almost positive from personal experience that it does.
Denkirson hasn't figured it out yet either :/
And as far as emptying half a clip...... you're either lagging or you can't aim. I can usually kill 3 armored targets with one clip from a scarab. Aim for the head. Headshots matter more in this game than most others.