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Glen, a 2X Barlow combined with the 15mm will work like a 7.5mm eyepiece. Having had a fast 12.5" Newtonian before , this will be more than enough magnification most nights - a 6mm will be overkill when the seeing is less than perfect, you might have 1-2 nights a year when it's worthwhile.
Optically, a generic Barlow with an average 15mm eyepiece will be outperformed by a real 7.5mm eyepiece, usually by having fewer elements (less scattered light) and sharper image.
The trade off is that you could use the Barlow with your other eyepieces, vs the cost of a singe eyepiece around 6-7.5 mm.
I've used Barlows before, and I don't own one, preferring to use dedicated eyepieces.
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