Girodin,
It's hard to say. Inertia guns require a certain amount of actual movement of the gun during the initial phase of recoil (before the action starts to cycle), so it depends on the whole package of ammunition used, extra weight on the gun, and the shooter himself. For inertia guns, the more you "limp wrist" the shotgun, the less likely a problem will be. Some shooters have more problems with the pistol-grip stocked M1's because they actually end up holding the gun a lot "harder" than a straight stock and resist that initial recoil more. Some people hang eight shells off a sidesaddle and have a really long mag tube and have no problems. I doubt a light approx the size of a 2-cell Surefire would cause a problem, though.
With regard to reliability of semi vs. pump, I absolutely agree. Using local 3-Gun matches as an example, the pump gunners have roughly 5-10x the "malfunction" rate of the semi-autos due to shooter-induced problems. The only reason the semi-autos don't have an essentually-zero failure rate is that from time to time a new-to-3Gun shooter will bring a semi that malfunctions constantly or falls apart (ie, "the front fell off")-- these are usually Remingtons or weird off brand clones.
This is not to say that pumps cannot be fast or reliable-- I know a couple guys who can shoot a Nova as fast as a semi, but they have trained a lot more.
-z