After more and more testing off HD video, I am finding the best bang for the download speed buck is sticking with qscale 10 and chopping the frames to 15 while sizing the image size down to hd480. Here is an example –
ffmpeg -i 051810Perf1.m4v -qscale 10 -s hd480 -r 15 -async 2 051810Perf1.flv
Using a standard resolutuon that is based on 16 seems to be the best way to go. There are so many intricate elements to encoding video to flv to try and make it decent quality, but still large while having it load quickly for people with various speeds of broadband!