I updated the spreadsheet (linked in the thread starter) to compare the difficulty of 645 and (6x,4)*. I calculated that 645 is twice as hard as (6x,4)*. Those results look a lot better. This method works better for patterns that are of a closer in difficulty level.
After some consideration, 5b cascade and 7b cascade are such different levels of difficulty that no real comparison can be drawn. Aside from the points that you raised, general physicality may be a problem for a lot of jugglers. Going over 100 catches with 7 balls requires quite a bit of physical stamina. I think that a (physically) average joe can juggle 5 balls for quite some time, but it takes athleticism (from training) to keep 7 balls in the air for longer than 20 seconds.
by Stephen Meschke, in response to this post 2015-10-31 19:45:01