[Pampered-Z]

Unless you’re running some type of competition header ( 1+3/4 – 2” Primaries with 4” collectors) or have small exhaust ports I wouldn’t worry too much about what you have. The cats will give you a bit more BP, after that the exhaust pulse is less of an issue.

I won't be all that concerned about the cat back. The weight of the system is probably more of an issue then the actual pressure, it is heavy!

Above 500 HP you really want a 4" system, the the key is actually in the muffler more then the size of the intermediate pipe being 3.5 or 4". You probably have either the Flowmaster or Spintech, the Spintech does have more back pressure then the flowmaster ( reason it's quiter too ). Both actually do produce a fair amount of BP, common for racers to switch to a moer bullet type stright thru design which will gain HPs.

So don't be too concerned about back pressure. In general terms: You need to worry more about too much back pressure then not enough. Not enough would be addressed starting at the exhaust ports where you want to focus on exhaust gas scavenging and velocity. Issues you could run into with small exhaust ports, cam with very tight LSA, or excessively large primary tubes.

Or address it with step tube headers. With a standard street type comp header it’s a none issue unless you want to spend a ton of money trying to find 5 more HP but tuning your headers? If you were maybe running some class that mandated none ported heads or stock cams where a few more HPs could be made by tuning your headers to get the exhaust scavenging perfect. This is where the exhaust pulse/velocity from one cylinder is helping pull the exhaust gases down from another cylinder. This continues down the entire exhaust system to some extent, the large the exhaust, the less velocity your have due to the volume, leading to the exhuast needing to be pushed out.