Mikey blogs about linux-next workload with pretty graphs. Ideally, we should all have our patches marshalled before the freeze, and there should be no pre-merge-window peak. I've gotten close to this in recent times, by being lazy and being content to wait for the next cycle if I miss one. Rushing things in tends to imply we skimped on review and testing.
So on that basis 2.6.30 looks like a winner: it has the smallest "peak of crap".
If you want to actually measure how much work Stephen is having to do, you would have to figure out what changes he is seeing. Approximately, that would be the changes in Linus' tree that did not come from linux-next, plus any new work entering linux-next itself. Anyone?