This paper presents two experiments investigating perceptual tolerances regarding deviations in the late reverberation of a room in augmented reality (AR) audio rendering. The study is based on binaural room impulse responses (BRIRs) measured with a KEMAR head-and-torso simulator in two seminar rooms with reverberation times (RTs) of about 0.4 s and 1.1 s. We implemented an algorithm to modify the RT while maintaining the spectral profile of the room’s reverberation. In a single stimulus listening test design, participants had to rate externalization, audiovisual plausibility, and room perception for different RT scalings. Differentiating between audiovisual plausibility for source and room helped capture the different perceptual phenomena. In this context, the concept of room acoustic signature preservation has also been proposed. The results indicate that in the reverberant room, RT deviation of 0.1 s already reveal that the acoustics of the room are different. However, plausible illusions in AR can be maintained despite significant perceptible deviations in RT, considering the original early room response. For originally short RT, audiovisual source plausibility is even robust towards larger RT modifications.