if you look around the GPU capability of RPI, you will know there is no issues with that and xbmc is smooth enough mostly is because of RPI's GPU. The problem occurs mostly when 5.1 audio/DTS decoding. May be the same reason i've not faced any of the theories you are talking here. Also i must mention my files are accessed using NFS from wdmycloud which is connected via LAN. So there is no bottleneck here as well. Again i'm talking all practical if you want me to test some files, i'm happy to download, test and post the results for you.... after all i dont know how i can efficiently use my unlimited 35mbps fiber connectionThere are actually two bottlenecks. highly compressed for sized YIFY's movies and high data bandwidth moviles like NLUPPER's.
YIFY's are highly compressed for size, ~1.4GB so they need less IO bandwidth but very high cpu to decode. a typical dual core celeron will max out decoding these movies.
on the other hand NLUPPER's releases are ~10GB and use very less compression and filters. so they use less CPU but need faster storage. I once played rango movie full hd and high bandwidth on a allwinner 1GHz chinese tablet without any problem. so it all depends upon how good are the codes for the particular processor. rpi's codecs and BSP (board support package) are not open source.
bottomline is that choosing correct pirated source helps a lot