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Barlow is generally best, less glass I guess. You can get a huge focal length with an eyepiece but I haven't seen too many people doing it well using that method (there are some exceptions).
A native FL of 2500mm is pretty long already, you'll likely use a 3x barlow to give you around 7500mm FL with an 8" scope. Big enough image scale.
You'll just have to judge it based on the object and the focal ratio and whether you're getting enough light into the histogram.
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