1.Yes the recoil will be different going from a comp to a flash hider, will it be dramatic, I know it is from my surefire brake to my old a2 birdcage. Adjustable gas block and low mass carrier do help, how much? not sure. 2.The flash hider might be "noticeably" quieter than the comp, but the sound is coming from the same cartridge so there isn't any more sound,the muzzle device redirects the exhaust gases to mitigate recoil, so to the shooter or others it appears to be louder when being in or near the direction of the ports. The thread protector is going to protect the threads, it is the same as and open barrel, so this will have the most flash and recoil as it isnt designed to mitigate anything. The flash hider as compared to the brake with have more recoil and less flash, and the flash hider will not have as much flash or recoil as a threat protector. My a2 birdcage has some recoil reducing capabilities due to the ports facing the top of the muzzle device. I put surefire muzzle brake adapters on both my 5.56 and 308 guns as recoil reduction was my primary concern. I hope this helps some, so many choices out there that I'm sure you can find exactly what you want.