Performance issues resolved
I took some time over the holiday to reinstall the last round of plugins, one set at a time, to try and trace back the source of my performance problems.
Thanks to the Oblivion Mod Manager (OBMM) - http://timeslip.chorrol.com/obmm.html - I traced the issue back to the ArchiveInvalidation.txt file used by many mods to point Oblivion to new locations of already used textures in the game.
Short story - forget updating this file and use OBMM to manage the load order of your mods, review and resolve conflicts.