My mechanic's thoughts on the Berk CBE & HFC

Speedy

I take it your mechanic is some older guy? Maybe not up with current times?

HFCs will give you power as well as your cat back, so don't fret.

Most of the HFCs I've seen for the Z have "J" pipes installed for the O2 sensors. This pulls them away from the exhaust stream like another poster mentioned. This "should" avoid the CEL, if not you can try some extensions called anti foulers.

With the tuning, the reason you don't see CEL is because the O2 sensors are usually turned off. That's fine from a CEL perspective, but if you have to pass emissions that will show up as a "not ready" sensor and you might fail emissions. Just be aware of that and hope the "J" pipes do their job.