Is this 400-500% true for more expensive operations inside of the block? It seems like he's just comparing the cost of procs to the cost of 1+1. I don't think the generalization has been established here.
Profiling is essential when figuring out how to improve your code. What we're doing here is explaining one weird benchmark result.
"Why blocks make Ruby methods 439% slower" implies something that isn't happening here, at least to me.