To answer some questions raised in this thread:
The internal working space of ACR and LR is Referred Input Medium Metric (RIMM), which has ProPhoto RGB primaries with linear gamma. Temporary excursions are made to other color spaces and image decompositions, as needed, to suit the image processing.
In ACR, there are additional options to set the desired output color space (which is used to drive the histogram and on-screen preview image). These color conversions are done at the end of the image processing pipeline, after the UI-driven controls (e.g., Exposure) are done. In LR, there are also such options (e.g., in Export and Web), though they do not drive the displayed histogram, nor the on-screen preview image. The color conversions are handled in exactly the same way.
Note that when you change a slider in ACR/LR (e.g., Exposure), ACR/LR doesn't simply recalculate the new exposure value. It also has to re-evaluate many other things. This is because your adjusted exposure may influence many other image processing parameters (for example, a brightness-dependent color table, brightness-dependent tone or color masks, etc.).