Also, say, I want to collect data about the performance of the web application. Can that be collected, or does it require explicit consent as well?
Also rember that one of the goals is to avoid illicit linking - so being able to verify that ip n.n.n.n is the same as slow_hash(salt+other-ip) won't fly as "not storing".
In general, anonymizing data in sparse populations is tricky - where "small" can be quite large. Just imagine building a bitfield of variables like: sex,age +/-50;2 bits. Rough location (easily 6 bits), browser (2 bits), mobile? 1 bit - that's already 12 bits etc. See also NYC taxi dataset, eg (not the article I had in mind, but seems to cover similar points):
https://research.neustar.biz/2014/09/15/riding-with-the-star...