I tried flipping the stem (which lowers the bars and increases the reach) for several hundred miles, but it makes my back ache. It's okay if I don't ride more than 25 miles, but no good for my solo centuries. BTW, in the down position, my bar is about 5" lower than my saddle. So I flipped it back up, so my bar is about 3" below my saddle-->I can always go on the drops if I want to go lower.

Remember that aerodynamics is about coefficient of drag and frontal surface. You wear tight, smooth cycling gear to address the first issue. For the 2nd issue (what this stem flipping is about?), you can do a couple of things, you can lower your head so it's level with the rest of your body, or you can raise your butt to the rest of your body. Either one will work as long as you try to have your body level, so if your saddle is not at the max power-stroke level, you can also raise your saddle rather than flip your stem. Just a different perspective...