I hear this all the time about back pressure..... I can tell you that I’ve been building engines, race cars, motorcycles, and exhausts for a long time. Having no back pressure will not hurt your engine nor will having to much of it cause your exhaust valves to not seat. All factory exhausts are tuned to run at a specific rpm and mostly to meet noise ordinance and emmisions laws. That being said, no back pressure will not “burn your valves” or “lean out” your AFR. It only means you have changed the rpms your engine will run best at. It all depends on length of headers/where the collector is and if they are equal length. But what it boils down to is the least amount of back pressure there is the better. Your engine is an air pump, faster air in and out is key while exhaust scavenging has a role in that as well. The only engines that need back pressure are 2 stroke engines, since they rely on a perfect exhaust pulse width to run correctly and don’t have valves (notice there exhaust shape) not being mean but read some engine performance books.