While it's true that some of the propelling gases are siphoned off to operate the action, the gas volume used is relatively insignificant. Furthermore, by the time the gas system unlocks the bolt and allows it to begin rearward travel, the bullet is out the barrel and pressures have dropped to a safe level. Thus, all else being equal, the difference in velocity between a gas operated semi-auto (really a bolt action operated by gas forces) and a bolt action is negligible.