Most-efficient way to copy iPhoto events between computers?
The most efficient way to accomplish this would be to merge libraries. Unfortunately, this is not included as a feature within iPhoto, but can be accomplished using an excellent program, iPhoto Library Manager. It's compatible with all versions of iPhoto v8.1.2 or later. 


Your iPhoto Library, by default, is located at: ~/Pictures/iPhoto Library
Either place the iPhoto Library of Computer A on an HFS+ formatted external HDD, or use Target Disk Mode to mount Computer A's HDD as an external drive on computer B. I recommend the former method. 





Choose Source Library: path to A's library on external HDD
Choose Destination Library: path to B's library.
See (3) and (4) for Duplicate Handling and Additional Options, then merge the libraries.