"Those of you who have an M8 or M9 and m4/3 systems, how well do the systems complement each other? Am I likely to see a significant IQ advantage to the M9 as opposed to my E-P1?"
I have an M9 in addition to m4/3. To answer the second part of your question first, yes you will see a significant IQ advantage. The M9 is easily the best image quality I have ever produced on a digital camera. The files are jaw dropping in terms of sharpness, colour and contrast. I sold a Nikon D3X to buy the Leica & if I interpolate an M9 file to the same size as a D3X file, the M9 file is sharper. The lack of an AA filter does make a difference & with the M9 Leica have "got it right" The M8 didn't quite "get it right" with the IR problems and some occasional strange colour shifts.
Dealing with the first part of your question and the original premise is a little more complex. I use m4/3 extensively & will continue to do so. There are three obvious reasons - The need for autofocus, the ability to shoot video and the possibility of using zoom lenses. None are possible with an M9. Also the M9 is a "hoover" as far as dust spots go. Its easily the worst camera I've ever used for this. m4/3 is excellent in this regard. If I know that I'm going to be shooting with a lot of clear sky in the picture then the thought of the time taken removing endless dust spots is uninviting to say the least.
I am also reluctant to take the M9 into what might be called "difficult terrain" There are waterproofing issues (lack of!!) with Leica digital cameras & I make sure that I don't take any chances with mine. I also don't use it in any situation where I might get knocked or jostled. Its an unfortunate consequence that I tend to be so protective of a camera that cost me an awful lot of money. I did think long and hard about buying it for that very reason & was concerned that I would almost be "scared" to use it in certain situations. To a certain extent that is the case, particularly as I managed to drop my M8, and destroyed the screen. Though this was repaired and replaced by Leica free under the passport scheme, it constantly reminds me that these cameras are far from indestructible.
m4/3 also gives me certain advantages because of the size of the sensor and the size/weight. I'm a full-time photographer. I shoot some commercial/advertising/event work for which I use Canon DSLR's but the bulk of my income comes from shooting travel and landscape for picture libraries. Much of this requires low ISO, good weather and maximum depth of field. A lot of my work involves walking substantial distances and the Leica, though small, is not as light as m4/3. A GF1 plus 20mm f1.7 and 45mm f2.8 is a wonderfully light option. Because of the 2x crop I also get the advantage of being able to use wider apertures than I would with the full-frame Leica with the consequent increase in shutter speeds. Since I work hand held this helps enormously. Some see disadvantages in the fact that it is often difficult to get selective focus with m4/3, however for me this is an advantage, since I usually want the most DOF that I can get.
I also get enormous enjoyment out of using m4/3. It was a system that I had longed for. Carting DSLR's and heavy lenses around for years has left me with a back that is prone to painful spasms. My consumption of pain killers has certainly diminished using m4/3!! They are also unobtrusive cameras. No-one takes any notice of me using them, unlike the big Nikons or Canons which seem to attract a lot of attention. Having said that the Leica goes almost completely unnoticed. Its only really photographers who know what they are. Most people just give me a "Poor soul, he's using his grandads camera" look.
For me the two systems complement each other. I have recently returned to using m4/3 a lot after a spell with the Samsung NX10. Though an excellent system the somewhat bulky zooms for the NX10 tend to take away the advantages of the light body. Though still a lighter and smaller system than the majority of DSLR's, it doesn't have the compactness of m4/3.
The M9 is a fabulous camera, and produces fabulous results, but m4/3 more than holds it own. I sell just as many pictures taken on my GH1 as I do pictures taken on the M9. So long as people are kind enough to buy my pictures in quantities that allow me to keep both systems, I will continue to do so.