1. Latency != bandwidth. The latency I'm concerned about is not only the hardware latency, but the task scheduler latency in starting the job. For even a fully uncompressed HD image at 1920 x 1080 ( 1 byte per band per pixel at 3 bands per pixel) the transfer time is going to be less than 1/1000 of a second from main memory to GPU memory on a PCI Ex16 V2.x bus at 8 GB/s. The difference in overhead between HD and low def is minimum in transferring because the constant latency time would still be there in HD. The payoff would be greater.
2. I don't have any data to back up my beliefs.
From the type of questions you two are asking me and the responses I'm getting on IRC I get the point that the developers are not too interested in this. I am interested in the Motion Estimation task in
http://wiki.videolan.org/SoC_x264_2010#Projects however I am going to take more time to fully explore the code in x264 before committing myself to it. I've only been studying it for 2 weeks.