How can I figure out if a Safari extension is using unreasonable memory/cpu? I'm not really experiencing any unreasonable sluggishness, but I am starting to pile up on the extensions and was considering pruning the most performance draining ones just for good measure.
Disable the extension. Run Safari for an hour (normal use) and note the kind of cpu/memory usages you get from it (using activity monitor or top).
Enable the extension. Repeat.
You can also use Instruments and attach it to your safari instance and start from there.