Well, the result are in. I tried Shell Optimax for 3 tankfuls, then switched back to normal unleaded for a tank to see what the differences were.
The result: I'm about £8 worse off than I would have been.
No noticable increase in performance, no improvement in economy.
Keep your money in your pockets.
Disclaimer: other vehickes may well prefer Optimax, this was a highly unscientific experiment and does not represent the views of the site owners, members, or (strangely enough) the author. :D