Obviously some objects really are more suited to large telescopes, e.g. the above M13 shot on a Canon 1Ds. Lots of sensor acreage wasted. 2x tele-extenders are not particularly useful for DSO imaging. The reason is that they make your lens go down by 2 f-stops, and then for unguided imaging you end up being restricted to star clusters and the brightest DSOs because too-short sub-exposures rapidly lose efficiency in stacking. 1.4x tele-extenders remain useful, but as newer cameras acquire more and more megapixels, and hence finer pixel pitches, even their utility is coming under threat. With the latest DSLRs having pixels smaller than 5 microns square, I suspect that in most cases it becomes more expedient to simply crop, rather than use a 1.4x extender.