In my opinion (which shouldn't be taken TOO seriously considering I only average 16-17), once you've got 2 Look Last Layer down, work on getting your PLLs fast. Once they are all below 3 seconds, or maybe even 2 seconds, you should start learning some algs for the more complex F2L cases (assuming you already use intuitive F2L). If you already know all the algs for F2L, then I think increasing your PLL speed as well as doing F2L only solves will help you get faster.
In terms of COLL and all that jazz, I would doubt that it's necessary to get sub-15. In fact, I would even guess that you won't need them period. Some that are fun to do are when you have a headlights OLL case, since the OLL sucks, maybe learn the different cases for that and turn the headache of headlights into an "I want headlights" because then you can get either the whole LL done or at least the corners done as well. Use a small group of headlights + corners and/or edges to force a smaller set of possibilities for PLL.
This is just my opinion though.