Believe me you do not want a 'softer shock'!! it would bottom out on every pebble you drove over. if anything, you would want a stiffer spring rate. As I have said many times... tires make a big difference!
Have an '08 and a '15. Only 26K on the '08, 1700 on the '15. They couldn't handle any differently. Don't know what they did to the springs and shocks on the '15, but it's downright civilized compared to the '08. And MUCH more stable.
Are we talking 2015 453 shocks and struts and shocks or 2015 451 shocks and struts. To expand on that are any of the 453 suspension parts interchangeable to 451 models in search of a less harsh ride.
Are you able to point to some of the suspension hardware changes that lead to the 453 ride harshness differences over the 451. Of course I am hoping for magical answers like the 453 front spring fits the 451 and eliminates all harshness in the ride lol...
If you put 'softer' shocks on there, you could get a rougher ride, and it could bottom out more often. Maybe you are expecting a ride similar to a car with much more suspension than the smart has. It will never ride like a Lincoln town car, or a Mercedes sedan. It just isn't engineered that way. Changing shocks and struts may give you a better ride, but handling might suffer. If the trade off is OK, then go for it!
I think you will need a bit more suspension travel to get a smooth ride over potholes and tracks.
Ride quality can also be improved by running lighter wheels. It's not as big of a difference as running a higher profile tire, and not even the same magnitude as decreasing the tire pressures by a couple of PSI, but there are many other benefits to running the lightest rims and tires you can.