Any reason why you want to go for a smaller tire width? I think the smallest you'd probably be able to fit on stock rims are 28s. 25s are pretty small and going to be a big jump for rims that should be holding 32s. Wider tires usually mean a more gentle ride and the sirrus sure is a sweet ride.

If you are willing to get new rims for smaller road tries, it really comes down to a question of how much you want to spend at that point.