Ok let's see if I can break this up..
Lowering: I've Lowered my car and the better handling and slight improvement to aero are nice, but neither make a measurable difference. The reason I lowered mine was because mine had tired old springs and shocks from 1994 and we're becoming a safety issue. Anytime something breaks on my car I see it as an opportunity to upgrade, but if it ain't broke don't fix it.
Belly pan: it's nice that yours has a belly pan to start with, because it will give you something to build off of, if you decide to do a full underbelly. The weight of the underbelly is definitely worth it. I have seen some people make theirs out of coroplast, others have used thin gauge metal. If your know how to fiberglass you could even go that route. The exhaust shouldn't be an issue. It is a very small percentage of the underbelly so even if you covered everything but the exhaust, you would still be much better off. My plan it to use coroplast underneath connected to a section of aluminum flashing where the exhaust is. Should keep the coroplast from melting.
Grill: Not sure how your grill looks, I'm going off of images from Google. If the images I'm looking at are correct, you could completely block the upper grill with a sheet of coroplast from behind and paint it black. It would still look stock. I'm no aerodynamic whiz, bit I'll do my best... The "slant" around the bumper that forces air to the upper grill would do its job and fill that cavity with air. When the area is full, the air pressure in that spot will increase and force all new air over and around it, as if that cavity was solid and flush with the body.
This is the image I'm referring to
