Ok the way I see it is this, I do notice a picture quality difference between the SL and DL encodes of my fake-anamorphic transfer, the higher bitrate of DL means the detail (grain in this case) can be 'described' (compressed) with more bytes and the graininess doesn't look as bad as on a SL transfer.

You can get round needing a high bitrate encode by smoothing out the grain so there's less detail to compress but this can also mean a slightly blurred picture (looks a bit out of focus) such as with the Dr Gonzo transfer. Throw in some temporal smoothing and it means even less detail to compress but the side effect of this is motion blurring.

Basically the cleaner the source the smaller you can compress the video and it still looks good, a noisy/grainy picture needs a higher bitrate.