Overhead of using Panels
This is the only thing I recall seeing, at DrupalCon San Fran in 2010. Sam Boyer, a maintainer of Panels, discusses benchmarks a bit in this video:  http://sf2010.drupal.org/conference/sessions/state-panels-3-and-more-what-you-love  at about the 29 minute mark.  It's not really a great benchmark test, but gives some idea into what page manager is running/processing.