1) You may be able to get away without tuning the car. Without tuning, it's not worth the $$.
2) Mild gains with proper tuning. Not to the extent of ELH, but you could probably bump up the timing a few degrees.
3) It will likely quiet your exhaust a little bit. With heat wrap (which you need), it will quiet it down more. Pitch, expect it to sound slightly higher-pitched. For the rumble, depending how unequal the length, it will become more prominent, stay the same, or slightly go away.
That said, if you're going to replace your headers, just go EL...