Configuring the display in 16 bit mode gave, on average, a 1.6x speedup in triangle drawing performance. The pixel operations, except for glDrawPixels, also showed improvements, to varying degrees.
One major caveat for 16 bit mode is that it also uses a 16 bit depth buffer. This is a very low resolution for depth buffering, and can cause significant errors if there is a wide range in depth values in a scene. Typically, errors will be noticeable in large scenes that require a distant far clipping plane, as illustrated below.
This snapshot is of the Virtual Harlem database in perfly, with a 24 bit display depth:
This snapshot shows the same scene with a 16 bit depth. The triangular and stripe-like artifacts are areas where the ground plane and the sides of buildings end up with the same 16 bit Z values, and so occlude each other incorrectly: