MF Digi-backs @ high ISO

I am searching for a medium format digital back that performs very well with low noise at high ISO (2500-3200) and long exposures (30-40 seconds). Does anyone have any experience? The largest sensor and highest pixel count would be secondary.


Thank,
Jeff

I don't think any MF backs exist yet that have low noise at ISO 3200. Most samples look pretty ugly at that speed. Could you use a high pixel count 35mm DSLR such as the D800E? I can confirm it is superb at high ISOs, but I haven't shot any long exposure shots yet. What do you need it for?