Fast Cast, Arts, Traits, and Cast Time Reduction

The playerbase currently believes Fast Cast and Arts are multiplicative. What is currently unknown is the manner in which other Cast Time Reduction (CTR) effects interact with each other. For example, due to the recast reduction on Divine Benison, it is believed that the effect is 50% Fast Cast. Does this effect stack with Fast Cast additively such that 15 Fast Cast and Benison grant a total of 65% CTR? Or are they multiplicative for a total of 57.5% CTR instead? How about Magian Staffs? Do they add to the Fast Cast level or are they multiplicative? It is currently understood that 80% CTR is the total, so this would help players sort their gear better.

Isn't it all -CT & Fast Cast stack, but Art's doesn't.

I admire your optimism in posting this. Never lose heart.

Never give up, never surrender right?

@Alvian, I have heard such claims, but I haven't seen any confirmations via testing. I have an idea for a test, but I that requires two people willing to join in.