A while back I ran a bunch of upgrades from 7.0 -- which 7.1 I believe was the first release with i2p. Initially it consumed a large amount of system resources to complete the initial i2p wafl scan. With some of the nodes, this was not bearable, so it was disabled. I have since enabled many of them, and did not notice any significant improvement after the scan completed.
I have left the option set to on for periods of a month or so without any repurcussions. Ofcourse I was only running NFS (no iSCSI or FC LUNS).