I am not married to a 2 lens kit either. If a really good 200-400/3.5-5.6 G SSM came out, I'd be very tempted!
Speed really helps. Our cameras have an extra sensor that really speeds up AF if the lens has an aperture of f/2.8 or larger. It makes a big difference. It also means that by f/4 the photos are very sharp and fully sharp by f/4.5 or 5.6. I rarely shoot anything slower than f/4.5. If I am really looking for extreme sharpness, I may stop down to f/5.6, but that is about it.
On that note, f/2.8 is expensive by 200mm. Anything over that and I know for a fact that I have to get slower. I could never afford even a 300/2.8, much less a 400/2.8. That is why I would love to see a new 200-400/3.5-5.6 G SSM with a 77mm filter. It would complete my system and it would be all I could afford. It would be about the same size as my 70-200 G SSM with a 2x t/c attached.
I am now reading that the a900 begins to produce poor results at f/11 and smaller, due to lens diffraction and how the sensor captures it.
On the wide side, I would love to try a 12-24mm zoom, but it is probably not practical as I am not big on really wide angle. The 24mm focal length is about as wide as I can really see myself using.