The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with the Standard Output Sensitivity method defined in ISO 12232:2006, the standard used by camera manufacturers.
By our tests, the X-E1's measured sensitivities are about 1/3 to 1/2 stop lower than marked, which is unusual for a modern camera. This means that for any given light level, shutter speed and aperture on the X-E1 has to use a higher ISO to get an image of the same brightness as an accurately-rated camera.
Noise and Noise Reduction (JPEG)
The X-E1 does a very good job of balancing noise and fine detail in its JPEG output, even taking its generous ISO ratings into account. Fine detail is retained impressively well at high ISOs where other cameras in its class are visibly suffering; even at ISO 3200 images looks clean yet detailed. Low contrast detail reduces a little at ISO 6400 but the images are still entirely usable. At the two highest settings (12800 and 25600), which are only available in JPEG mode, detail degrades visibly. These modes should probably be reserved for smaller output sizes.
The X-E1 offers five noise reduction settings, which cover a good range of options between reducing noise and maintaining detail. If you want grittier, more detailed output you can set NR to '(-2) Low', while if you're after the smoothest output you can set '(+2) High'. There is little difference up to ISO 400 but at settings higher than that the variation in noise and detail levels becomes more obvious.
Raw noise (ACR 7.4 RC noise reduction set to zero)
As with the the X-Pro1 Adobe Camera Raw's processing of the X-E1's files looks radically different compared to its output from conventional Bayer sensor cameras. Chroma noise is strikingly low, and detail retention is impressively high - very much like the camera's JPEGs, in fact. Because of this, direct comparisons have to be treated with a degree of caution - it's best to assume that the de-mosaicing process of the X-Trans CMOS sensor behaves as though it's doing substantial chroma noise reduction relative to ACR's standard treatment of Bayer sensors. Again we also have to factor in the camera's over-rating of its ISOs.
That said, the output from the X-E1 is remarkably good at high ISOs, and it's difficult to see APS-C Bayer sensor cameras quite matching it at ISO 6400 even with substantial noise reduction applied.