It sucks, but the header is a big deal. The cat is actually not the real problem. Cutting it out doesnt help much.
You can see the problem as soon as the new header arrives. At that point, you see the header tubes. One set looks like its from the ninja 300...or 250. Very small diameter. I believe they are 32mm. The Arrow tubes are 40mm, as i recall. Massive difference in diameter. Weight, too. The stock header is very heavy. I believe the weight savings, for just the header, is close to 10lbs.
They are so under sized that you dont even lose low end and midrange when you switch to something larger. Its pretty obvious Kawasaki wanted this bike in that 125hp level.