I've upgraded to the 16 tooth sprocket and yes, I like it a lot - no longer am I constantly looking for a 7th gear. It has taken just a little fizz out of it's acceleration though, so if I spent all my time racing around city centres I'd leave it on 15. Would also not consider it if I was carrying pillions.
I couldn't find any info online about how to actually change the sprocket, so here's a how-to in a single sentence: the chain can stay on but you need to loosen the rear wheel right off to get enough slack, the gear selector needs to come off to allow the sprocket cover to be removed (so mark the position with paint/permanent marker), and the 30mm bolt needs torqueing to 145Nm afterwards.