Implemented JHFA102 and the above parameter reports correctly, strangely I had to reboot the node twice.
Also SNMPv3 user seemed to screw up, I had to delete and then add this back in or SNMP details would not be discovered.
In var/log/messages I have been seeing lots of the following since implementing the Jumbo:
kernel: dst_release: dst:xxxxxxxxxxxx refcnt:-xx
The above is explained in sk166363, which implies it can be safely ignored. This said I'm still going to ask TAC about it t cover myself.
So far I also believe CPU usage pattern has changed for the better but will tell tomorrow during a working day.