Whatever you're rendering, you probably aren't benefiting from PRMan's really distinguishing features if you're using blender.
For the average Blender-user, that's probably true, but certainly not because
PRman has historically been incredibly slow in comparison to other production renderers for almost a decade
... since most Blender-users are not going to be rendering multi-billion polygon scenes with massive displacement, instancing, complex shader networks, complex AOV output and custom-written Renderman shaders.
Sorry, but your statement is just completely wrong!
You mean the VFX companies who made Avengers: Age of Ultron, Cinderella, Ex Machina, Fast and Furious 7, Inside Out, Jupiter Ascending, Jurassic World, Mad Max: Fury Road, Seventh Son, Ted 2 and Tomorrowland thought they'd chose an "incredibly slow" production renderer, rather than Arnold?
Most people think of the REYES rendering approach as being at the core of RenderMan when thinking of imagery produced by Pixar's RenderMan, and although this has been historically true, RenderMan has been a hybrid renderer for some time now.
For the record, Renderman now uses RIS: Arnold-style path-tracing as well as bidirectional path tracing.