HP limit with 1-5/8 headers

It's difficult to give specific numbers, but the 1 5/8" primaries and 2 1/2" collectors will definitely restrict your HP. Personally, I've always found it better to be on the conservative side. I have TTI stepped headers that go from 1 5/8" to 1 3/4" with 3" collectors and I can run in the mid-11's while weighing well over 3,500lbs! Different weight-to horsepower calculators give different #'s. Not sure what the actual HP is, but it's up there. The step gives the best of both worlds - bottom end torque without being restrictive at higher rpm's. Some custom race headers start out about 1 1/2" or are shaped to the actual port opening! But they do step up quickly to 1 3/4" and/or, sometimes, 1 7/8", depending on the application. You will see a benefit by getting rid of your existing headers.