Fi = Fast Intentions exhaust system. It also could mean Forced Induction depends on the context.
Cbe = Cat back exhaust.
Nice sound clips OP! I love the FI CBE! But I think there is another CBE is equally awesome but people here hardly talking about it. That's the HKS Hi Power. Different exhaust note tho.
I had FI Carbon Fiber CBE on my g35 with HFC. It's not overkill the sound. But I heard FI CBE with the test pipe will sound even better!
Fast Intentions Carbon Fiber Exhaust Vids+Pics - G35Driver