I prefer DxO Optics Pro for managing ISO noise during RAW conversions.
.. the problem being added hd space because dxo exports only linear tiff afaik - while inside lr everything is done on the much smaller raw image. But dxo is known to have the best lens correction profiles for sure.
This is ISO 12800 on the 1D X, and the DR isn't bad, considering...
Impressive in comparison to my 60d, but still the blown highlights on the cap look distracting, at least to me.
Getting a correct expoosure and avoiding underexposure is critical use higher ISO to get a good exposure. There will be less noise than you get from a underexposed image.
The big issue with low light and high ISO shooting is not noise, but lack of DR. A dim light on one side of the stage can be blown out while the rest of the image is underexposed. Don't be afraid to increase ISO enough to slightly overexpose, pulling down the exposure 1/3 stop in Lightroom decreases noise in the dark areas.
Um, did you just contradict yourself or I am misunderstanding you? The problem with lack of dr is mostly that you get blown highlights, so a bit underexposing might be safer unless the scene has very low contrast. And raising iso instead of underexposing even further lowers dr.
And to make it really complicated, the "best" iso depends on the scene, have a look at what the Magic Lantern dev figured out (you can customize iso much more with ml): http://magiclantern.wikia.com/wiki/ISO#Then.2C_what_is_the_best_ISO.3F