During the few years I was making my pathetic attempts at Bonneville Salt Flats, I had the extreme pleasure of becoming friends with Tom Burkland, former piston powered, wheel driven land speed record holder at Bonneville (417 mph in a twin engined, 4 wheel drive streamliner, reaching 450 mph in one pass - an all time, piston powered, wheel-driven record speed). When discussing my poor pathetic efforts with him, his advice was to chose:
either slam the car down as close to the ground as you can (with a front air dam that also almost drags the ground) - the idea being to try to keep as close to ALL the air out from under it as you can,
or;
streamline the underneath with a belly pan just as smooth and aero as the top & sides.
Now, at Bonneville, there are rules out the ying-yang, and as I soon discovered, all the stuff that would REALLY help was also illegal!
The car I was driving (actually, the CLASS I was in) did
not allow belly pan streamlining. Therefore, the consensus was to get it as low to the ground as I could. HOWEVER, Tom's streamliner was allowed to have streamlining anywhere and everywhere, so he did the belly pan. (and it was pretty low, too). As I recall, if you have a choice, do the belly pan. Lowering is just the best you can do if you're not allowed a belly pan.
So after all that, I think I would have to concur with those who say "do the steamlined belly pan & you'll get more". Lowering should help too, but it's a weak, second-place choice. (The car in the Hot Rod article was in a class similar to mine, where belly pans were NOT allowed, too)