Doubling your FL ( e.g.to 1200 mm for example) will indeed give you a larger image scale (x2) but at the expense of 1/4 of the intensity of light falling on your sensor so it will take X4 exposure length to capture the same intensity of image as at 600 mm.
Easily done with a X 2 Barlow but be aware that increasing the FL will double the difficulty of tracking or guiding and with mount drive errors,wind,balance, and polar alignment will give you double the trouble of those small errors you may see at 600 mm.
I would say to exhaust the possibilities of the larger DSO's with what you have now, and there are many of them coming around every month.
I don't believe that moving from 600 mm to 800 mm will satisfy you (it wouldn't for me !).
Those tasty little galaxies need at least 1200 mm and $$$$$'s!