Strangely enough a copper plug usually has a fatter spark than an iridium, or platinum.
They just don't last as well, which on a high rpm motor can be a problem.
Due to the frequency of service I did run copper in my SYM until the last change, but run iridium in my Burgman, which has a longer service interval and is also more of a PITA to change.
Back when I had my 2012 GTI 300 some folks were running a iridium spark plug for better performance. Are they still the better option for a spark plug in a scooter?