Best not to use the in-camera long exposure NR – doing so actually *adds* random noise. It's good for removing hot/stuck pixels, but RAW converters do that anyway. When subtracting a dark frame, the random noise is in different places in the two frames, so you're effectively summing the noise (the way to remove random noise is to average frames). The noise addition with LENR isn't really noticeable at low ISO, but at higher ISO settings like 1600+, you'll see it.