Probably things like spectrograms, waveforms, etc. For a lot of effects it doesn't really pay to GPU accelerate their drawing - in this case looks like REAPER is using custom drawing code into an image, but the compositing these image widgets together with native Apple drawing functions onto the final window plane. Doing the final compositing step with the GPU and shader color conversion is one option. Two other options I'm surprised weren't mentioned were drawing the image widgets in the final colorspace to avoid the conversion step, and specifically requesting an 8-bit sRGB window for system compositor-side conversion (I'm not sure if OS X supports the latter).