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To get back to the OPs query. An 8" scope has a "theoretical" maximum power of about 50x per inch which sounds good as it yields 400x, so why not just go and buy a 3mm EP?
Unfortunately for most scopes theoretical and practical power are vastly different and in most cases practical power is more like 10x-20x per inch or around 80x-160x. In good conditions that can be pushed higher. Limiting factors include quality of the optics, focal ratio, seeing conditions, accuracy of collimation, optical quality of the EP, among others.
Increased power does have the advantage that it darkens the background sky, technically it is increasing the power past contrast threshold, but that must be balanced against the other factors mentioned about in particular seeing conditions.
Most deep sky objects will look very nice at less than 130x from experience, so unless you really need higher power I wouldn't bother with a barlow. If a 10mm is your only EP I would look at getting a lower power one first.
Cheers
Malcolm
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