and get the bolt speed up to fast you run into the problem of out running the magazine springI read about shooters trying to increase bolt cycle velocity so they could get faster follow-up shots. When you do the math, one needs to have a reaction time faster than 1/30 sec to benefit from a faster than stock cycle time. So, in my economy, the only folks making out are the ones selling lightweight carriers, and all sorts of tweaked small parts to make such a gun cycle reliably.
Regarding Adj manifolds, there is a benefit when using a suppressor- but not as much as some would suppose. The bullet is gone, BCG is beginning to move and the system is bleeding gas to ambient at a fast rate from three points- muzzle of the can, the breech and gas port. The trade off is once you set it for your load, your rifle may not be reliable with any other ammunition. A bit if extra gs bleeding out the breech is not something to die for, hobbyists, well, that’s who all the boutique stuff is made for.
and get the bolt speed up to fast you run into the problem of out running the magazine springThat’s why the AR is so popular. It’s a platform that people can tune, tweak, finagle, adjust, and fiddle with to their hearts content.
I’m of the mind that if it ain’t broke, don’t fix it. With a caveat. I was on the state shooting team for 3 years and I absolutely have to have a good crisp match trigger. Other than the trigger, every rifle is a tool with a job.