Restoring an old backup to latest MySQL release
IMHO, when it comes to MySQL 5.5, you will definitely see much better performance from InnoDB than in past releases because dirty pages in the InnoDB Buffer Pool do not linger as long. Those dirty pages are flushed more robustly.

You also have the luxury of creating multiple InnoDB Buffer Pools (See innodb_buffer_pool_instances and innodb_buffer_pool_size). To reduce thread locking amongst the buffer pools, make sure set innodb_thread_concurrency to 0 to let InnoDB storage engine decide best how to handle thread allocation.

CAVEAT ON MIGRATION TO MySQL 5.5

Make sure you mysqldump all databases EXCEPT the mysql schema. There is a cleaner, crisper way to migrate User Grants out of the MySQL Schema. In fact, there are two options for accomplishing this:

OPTION 1 : Use mk-show-grants

This dumps out all the MySQL Grants as SQL statements, which is completely portable to any MySQL 5.x instance.

OPTION 2 : Run these commands (My personal emulation of what mk-show-grants does)

mysql -hhostaddr -umyuserid -pmypassword --skip-column-names -A -e"SELECT CONCAT('SHOW GRANTS FOR ''',user,'''@''',host,''';') FROM mysql.user WHERE user&lt;&gt;''" | mysql -hhostaddr -umyuserid -pmypassword --skip-column-names -A | sed 's/$/;/g' &gt; MySQLUserGrants.sql