this would be very noisey to my tastes. hence the use of low iso on my cameras.
im not professional. but i would not keep anything that shows a hint of noise. is too distracting for me.
i guess i will have to rent a lens a try it for my self.
The 1DsIII will be quite a bit worse than my shot (remember that's a 12800iso shot on D3s so 6400iso on D3). It's also a 100% crop with not lumi NR added. So it can be cleaned up if noise is distracting. Personally I don't mind the grain, it's nice and smooth (unlike the grain on the 40D) and adds character (YMMV).
I wonder though, you said a 50d shooting 1600 was impressive to you. I'd imagine 1600 on a 50D would be at best like shooting at 3200 on a D3. Shooting at 800 would therefore be super clean for you.
Really it comes down to the MP.