I had the same thing on my Nismo. Berk cats first with Nismo cold air intake and uprev tune. I then added the FI exhaust weeks later and had dyno pulls done, and not retuned as they were booked up. It did lose power on low and mid range and labored as you noted. The dyno sheets showed it and I felt the power loss. So you really should get uprev to tune it. It can't adapt that much.

I want this same exhaust! ive heard it on some g37s and other nismos...how long did it take to ship?