The following graph shows the performance differences for 'select' and 'insert' statements using a PBMS enabled version of InnoDB on an 8 core machine.
The test compares the insert and select performance of LongBlob columns with PBMS support against that of varchar and longtext columns when using InnoDB.
The test shows that depending on if your application is more heavily weighted towards Inserts or selects it may be beneficial to replace columns containing more than 10K of data with longblob columns with PBMS support. In all cases the performance of both 'selects' and 'inserts' was improved for columns containing more than 20K when using longblob columns and PBMS. As the data size in the column increased the performance gain by using PBMS also increased, at 10 M there was %580 performance improvement for selects.
The testing showed some performance irregularities with PBMS which when fixed should result in even better performance.
If anyone would like to try this themselves please contact me and I can give you a copy of the test tool I used as well as a PBMS enabled version of InnoDB.