Re a previous posters comment, a 25mm will make sod-all difference to aerodynamics. I prefer 25mm tyres because the bigger air pocket means that you can run lower pressures and hence get more 'rubber on the road' in comparison to a 23mm, so does improve handling in corners. I use a local circuit for training with a fast 180 degree bend - people crash regularly in races - and you can test the limits of adhesion and handling. A 25mm tyre is more predicatble and doesn't feel like giving way when you heel it over. A PR2 in 25mm would be a great tyre for RRs on typical UK roads