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I find the 2mm exit pupil rule spot on for me when it comes to faint fuzzies. Anything under 1.5mm exit pupil is too little light IME, except for brighter objects. (exit pupil = EP focal length / telescope f-ratio) But they are called faint fuzzies for a reason. In an 8" scope most galaxies will be faint wisps and/or puffs of light. You need dark skies, dark adapted eyes, and some practice to see structure. Couple of the best ones in the night sky atm are NGC 253 and NGC 1365. In an 8", you can see mottling on the disk for the former and the barred spiral arms of the latter.
One DSO EP for f/10 (I assume) for around $300? I'd go for an LVW 22 or 17mm. (and I'd eventually get both.) But if you already have a good 17mm Plossl, do not expect miracles from a more expensive EP. Most of the gain from a more expensive EP will be in a well corrected larger FOV not in detail seen.
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