I'm not Bob,
But I was a sound engineer in one of my past lives. Bob's right; differential mics do exist, and have for a long time. Find some old video of Grateful Dead concerts from the '70s when they had their 'wall of sound' PA system. You'll see two identical looking mics in front of each vocalist, one above the other. All the PA speakers fired directly into both mics, but the singer only sang into the top one.
Now, that technique is pretty rare in the 'mobile' world; not sure I've ever seen a headset that used that technique. Virtually all current headsets, and most vocal microphones used in live PA type settings, use 'unidirectional' mics. This link
http://www.soundonsound.com/sos/sep00/articles/direction.htm
has a pretty good diagram of how one works, about 1/2 way down the page. (Or just google the term; you'll learn more than you ever wanted to know.)
Reading your original post, you kinda have the solution embedded in the question. The headphones block the high ambient noise, but the mic can't help picking it up. (Unidirectional mics do have their limits.) Turning down the mic gain is a good suggestion; it increases overall signal to noise (ratio of the mic's perception of your voice's volume to its perception of the ambient noise. (Voice gets louder than the noise, from the mic's perspective.) Next step is reducing ambient noise, as you suspect, but the biggest offender, and easiest thing to fix, hasn't been mentioned.
As strange as it may sound, in most planes like RV's the biggest contributor to noise that affects communication is wind noise. If you haven't already done it, spend whatever time it takes (shouldn't take much money) to seal up all the air leaks around the canopy, wing roots (aileron pushrod boots, etc) and into the tailcone. I'll bet you'll see a dramatic improvement.
Quick test before taking any action: What's the noise/intercom squelch like while doing a runup on the ground? If it's significantly less bad, you probably have a wind noise issue.
Hope the above is useful,
Charlie