You won't see a change.. we do huge photo processing (files over 240MB one pic, sometime 300 pics at the same time) and tested it on MacPro and Macbook Pros, no difference noticeable
I just started the encrypting process and it seems like it's going to take a long time. The estimated time of completion is being erratic between 8 and 11 hours.
I'm on an early 2008 MPB. 2.4Ghz, 4gigs of RAM with a 500gig 7200rpm HDD.