Obviously it depends on what exactly the backup mechanism chosen by the designers of SQL Server is. On platforms which provide media to media transfer without passing through the computer's RAM copying from disc to cache and then from cache to backup media is inefficient - - but I have no idea whether any version of windows can use that feature even when hardware provides it so it may be that pushing stuff out to disc so that the copy process doesn't have to alternate between disc and RAM as source won't help by maximising the benefit of autonomous copy, and even if Windows can support this there's no guarantee that SQLS would support it. It is however true that even without autonomous copy it is faster, on some platforms, to arrange things so that very large chunks are written rather than a larger number of small ones, and in that case it's still possible that making it possible to transfer everything from disc and not refer to cache may give a performance gain; but again SQLS may have chosen not to do this as it may not be worth the effort (it may even be counterproductive on some platforms).