You are correct. There is about 30 titles with the full bitrate DTS encoding at 1.5mbps. Most of these titles have been re-released later with only 756kbps (to make room for extras on the disc). You are also correct, the legal limit for Dolby Digital is 640kbps but it was never used. DTS could also do 24bit/96khz but I have only seen 1 disc with that. There were also a handful of titles with DTS 6.1 (at 756kbps). I actually own one of these commercial DTS encoders (DTS CAE4/CAD4) which was professionally used to author consumer DVDs. These are such niche cases/numbers, I didn’t want to make my explanation larger than it already is. Nevertheless, they are all highly compressed and lossy soundtracks with the exception of 2ch LPCM soundtracks.
If you randomly pick some SD titles from the store and compare their aspect ratios (whether letterboxed or anamorphic widescreen), audio soundtracks (including commentary tracks), subtitles, included extras and the download file sizes, you will see that they all match exactly to their DVD versions. This is also true for the majority of the HD downloads on the store. There are some exceptions, typically on the titles released in the last 2-3 years. Maybe in the earlier days of the store, they were using the physical media masters rather than the mezzanine file from the streaming versions.