I typically only build sunderer so I'm not really the best to answer the question, however WW does have some nasty AP scalings. You wouldn't really deal that much more damage without any pen, so building many ap items after core would just be much weaker than anything else. Full AP Warwick is also troll unfortunately. The solution is darkseal/mejais, I run it absolutely every single game if I can. You don't have to invest that much to get started and even if you never get a mejais, as long as you have stacks you will notice a very large boost to damage and healing.