SQL Performance Tuning / CBO Training

I am exploring options to improve the understanding of one or two Oracle DBA's with respect to more in-depth / advanced SQL performance tuning. I am aware there are many excellent online resources freely available (and the paper versions) and nothing in my opinion would trump on-the-job experience, however we have a training budget available and I'm curious to see whats available out there.

Would anyone please be so kind as to offer reccomendations/advice on any paid training resources (Preferably available online), else within the UK, with regard to advanced SQL tuning or CBO performance?

What seems like an applicable candidate, of which I have uncovered thus far is from HotSos for Advanced Optimizer concepts (I'll avoid directly linking to the page here).

Any suggestions would be most appreciated.