A lot of the inspiration for how to actually make this work came from Mike Ash's awesome NSBlog:
I'd say the app takes around 2-3x less time to load than when we started our most recent performance quest, largely due to issues we found using hookshot. We did find some performance critical inner loops in our rendering code where we were able to achieve something like 10x speed-ups.
Apple's profiler has significantly lower overhead than hookshot - which makes sense given the relative amounts of information collected. Our app runs noticeably slower when hookshot is enabled, but when it is disabled, hookshot introduces exactly 0 overhead. We only enable it when we're specifically debugging performance issues.
Another example: we used the "HOOKSHOT_TAG" macro to tag objects related to rendering different days in Cue (today, yesterday, tomorrow). This makes the thread activity graph use different colors for time spent on those objects. This helped make obvious a few edge cases where some rendering/processing for a day that was not on screen was done before the day that was.