Same CPU speed in both modes is quite obvious, as game logic must always run in the same mode (at most they could cut it by a few tens MHz by suspending unessential non-gaming services that they possibly chose to run in background while docked). GPU speed and so graphics level is obviously where big power saving can be achieved when in portable mode, and this too shouldn't be matter of arguments.
What can really disappoint people with sound reasons is the choice of top speed when docked.