Revealing mistake: When Benji imagines putting on a mask of an agent he plans to impersonate, when the camera pans around to first show the mirror, the mirror Benji's hands are further up on his face than the reflection, and the mirror Benji's hands move down his face slightly before the reflection starts to move them. Also, the mole that Ethan has on his left cheek isn't reversed in the mirror image. The "mirror" was actually a hole in the wall between two identical sets (one mirrored), with Simon Pegg and a Tom Cruise double in the foreground, and the mask actor and Tom Cruise in the background "mirror" set. (00:57:20)
Answer: He most likely could have waited, but as you pointed out, it's more dramatic that way. This is a movie and the action is choreographed for maximum dramatic and visual effect, timing, tension, continuity, audience attention, and character interaction. We're not supposed to overthink it.
raywest ★
They would want Benji to enter as soon as possible, as the security may notice eventually that there is something amiss. Still going in without bothering about confirmation seems unnecessarily risky.