Is there any reason not to use Percona 'innodb_fast_checksum'?
The wording you pasted is very specific:


  The original algorithm is checked after the new one, so you can have data pages with old checksums and data pages with new checksums. However in this case, you may experience slow reads from pages having old checksums. 


This means that it can still read the old checksum, but from now on in, only Percona Server will be able to read your data.  This is later noted here:


  Once enabled, turning it off will require table re-creation as well, since it will fail to start on data files created when innodb_fast_checksums was enabled.


You've got to put it in context - InnoDB only verifies the checksums when it reads a page from the block storage device, and updates it before flushing it back to the storage device.  While in memory, the checksum is not maintained.

For many years, an IO to a disk has taken something on the order of 5-10ms (1ms = 1/1000th of a second).  Computing a checksum probably takes somewhere around 50us (1us = 1/1000000th of a second).  I don't have the actual data what it is in InnoDB's case, but if you Google "Numbers everyone should know", you'll hopefully agree I'm correct within an order of magnitude.

Enter an era now where we have things like Fusion-io flash devices, which on paper have a ~20-30us access time, and you can see that reevaluating the checksum makes sense.

My general advice: Don't ruin your backwards compatibility with MySQL releases unless you really need it.  Most people do not need it yet.