It has been a very very long working week-end for the technical team at Believe...
From latin1 to utf8 very few indexes as to be modified, reaching the max index column size limit of 767 bytes for InnoDB. Some Url like Referer as been change to varbinary as the column content was already a mixed of encoding stored in latin1.
Don' forget UTF8 is bad for in memory workload as strings in records and indexes will take much more space in memory buffers.
Thanks to the all team at Believe and to Laurent for the fantastic drive of the team building spirit.