Welcome! It's a great little car, you're going to love it.
1. Not that I know of. I've got some coroplast that I'm planning on blocking the whole lower grille with. I'll watch my temps and adjust as needed.
2. % increase is affected by other things. But it'll probably help.
3. Honda part #74111-TM8-A00 is a belly pan for a CRZ.
Apparently it will go on a Fit just fine, but it's behind the grille block on my to do list.
4. Great idea. Very low payoff, though. It's for after the big stuff is taken care of.
5. On top of the steering column can work well.