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Are your stock cans hollowed out or ‘gutted’ as some describe it? The tubing inside OEM is crazy constrictive - it consists of multiple very small diameter tubes that reverse the direction of the exhaust gases, passing them back and forth inside the OEM cans. If you haven’t cut out all the plumbing inside the OEM cans and replaced it with straight-through perforated tubes (with packing around the perf tubes), there will be too much back pressure to take advantage of the headers’ performance. Below are some photos of what you see when you cut open OEM 6 gen mufflers. Gases come in through a single inlet pipe and are immediately pushed through a perforated metal plate that is perpendicular to gas travel (A giant flow clogger). Then the gases push through two tubes into the middle chamber, then gases that are too willful to meet Honda engineers’ expectations reverse course through other tubes to circulate at the front of the can, then reverse course again to mix again in the mid chamber with gases that have remained in the middle chamber. Finally it’s all pushed out through the two little exit pipes we are familiar with. Here’s the screen over the inlet pipe way down at the bottom of the can (after all other midsection crap has been removed): Here’s the skinny little inlet pipe after removing the screen: Here are the noodly back and forth tubes in the midsection - now we know why OEM 6 gen mufflers are SO quiet! (Note the perforated sound damping material lining the inner walls of the can): ShipFixer, I’d suggest running the Delk until you find an open flowing slipon assembly with aesthetics that please you. Back pressure is never good with quality headers. Some say back pressure is a necessary component of a tuned exhaust, but the way it was described to me is that the column of gas moving through the exhaust system (Specifically after the final merge/collector) needs to have as little obstruction as possible. In a single muffler system, sometimes a longer canister works better, not because of back pressure, but because the column of moving gas spends a longer time moving through a controlled path, which in some cases allows the timing of exhaust pulses in the merges to be optimized. An example of this is my Aprilia RSV4 - another V4 - on which I had dyno runs done back to back, first with a 300mm long Akrapovic can then with a 420mm long Akrapovic. The cans were identical in all dimensions but length, and the bike put out 1.2 more hp and 2.3ft/lbs more torque with slightly richer fueling on the longer muffler. YMMV.