Can memcached improve performance for InnoDB in mem with a large innodb_buffer_pool_size?
The InnoDB Buffer Pool is the home of three classes of informational structures


Table Data Pages that have been accessed
Table Index Pages that have been accessed
Insert Buffer Changes to Update Secondary Indexes in ibdata1 (System Tablespace)
See Pictorial Representation of the InnoDB Buffer Pool and ibdata1


If you make the InnoDB Buffer Pool large enough, then all data you have accessed will essentially stay in RAM. Any data you have never read but finally decided to read will then have data and indexes cached in the InnoDB Buffer Pool. Please read my post : What are the main differences between InnoDB and MyISAM? on how to determine the size of a good InnoDB Buffer Pool.

Given a large enough InnoDB Buffer Pool, I would rule out using memcached.