| Dan,
not really any increase in performance to be gained from plugs and oil, just smooth running and longer lasting engine !!
I would advise to stick with the manufacture spec re oil and plugs on your cooper.
re performance I would advise that you just keep your cooper well serviced and save up for an S !!
my reason for that is I think you could end up spending a lot of money on your cooper for relatively little gains in performance.
again Dan, my opinion only and I also have a cooper, but hopefully you will get some other posts giving a different view.
good luck | 
