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I understand tracking in order to avoid star trails ... but how does tracking help to improve noise / ISO relative issues?
I also understand (haven't tried yet) stacking @ 30s intervals as a technique to minimize the effects, but that still doesn't suggest whether it's better to shoot base ISO, 800, 3200 or 1,000,000 as an optimal "trade-off" point @ noise vs. exposure.
Because a tracking device will allow you to use longer exposures without causing star trails, and it's the length of exposure (not ISO) that principally determines how much noise an image will have. Exposure is defined by how much light enters the camera, thus ISO is not a component of exposure. In fact in most cameras will have less noise at higher ISOs, provided the exposure is the same. This is due to how read noise (electronic noise) is optimized in the sensor/ADC for the gain applied for ISO.
So I guess the question is at what point (ISO/aperture limit) does one feel compelled to find faster glass than 2.8 @ wide or 2.0 @ tele is necessary for astro work ... in order to retain "clean" images.
Most large aperture lenses suffer from coma at their/near their largest apertures, so that's typically not the best solution.