I doubt that you will notice much difference. Are your tyres XL rated? The 225/40 18 should be 92Y which is the extra load rating.
We are happy on the 225/40 18 tyres for comfort. Better than the old Aero suspension anyway. I enquired about the size with Hirsch, because the older 2002 9-5 I had was originally on 225/45 17 tyres, whereas the Dame Edna came on 235/45 17 tyres. They mentioned the clearance on a loaded car as the main issue, but they also mentioned that, being a bit smaller in diameteralso helped the pick-up at low speed, but I think that would be very marginal. But you may notice the difference with a 4% drop in diameter