Only you can answer this question because only you know what you
consider professional standard photography. What size output are you thinking? Prints? Web galleries? Albums?
Fortunately it is very easy to set up a test, just go to a similar EV room and run some shots at different iso's, make sure you have some detail in there too. Software makes a huge difference as well, I hate the NR in Lightroom, it is junk, but take a file into PS or Topaz DeNoise3 and you will realise you can get away with much more than most people think. But again, only you can say what you are happy with.
I use 1Ds MkIII's, so essentially the same sensor as the 5D MkII it maxes out at a native 1600 iso native H is just 1600 pushed one stop in post. So I did a series of tests, here is the camera exposed three stops under at 1600iso, an effective 12,800 iso, then I raised the exposure three stops in post and did NR in Topaz.
Are the results junk? I depends, I have printed it to 8"x11" and the results are surprisingly good, it isn't a 1Dx
But I know what I'd get if I had
to do it. You need to do similar tests for your own satisfaction.
P.S. I never shoot above 800iso, that is my comfort zone.
Shot 1, full image shot at 1600iso and -3 EV for effective 12,800iso
Shot 2, 100% crop of imported file
Shot 3, 100% crop of exposure adjusted image
Shot 4, 100% crop of exposure and NR adjusted image