As someone who configures and maintains F5 boxes, I'm totally on board with Hamish's statements about re-encrypting not being necessary. With that said though, I also work in an environment where our security team has begun requiring it in certain areas. As long as physical access control is there, it's not a PCI requirement but re-encryption is almost considered a last line of defense in case someone internal goes rogue.
Of course, if that's a true requirement, there are still opportunities to minimize the performance impact. As Hamish also mentioned, using a shorter keylength can be a great option. We have an application that cannot support larger than 1024-bit. Since CAs now require 2048 or larger, LTM essentially allowed the application to continue working.