I have no experience with the SSL kit but here we have a lot of satisfied users. On my 2014 4/4 I have changed the dampers with AVO ones all around which are adjustable. And I have 140 pound springs of Mulfab in the front as well as the 4 seater 4 leaf springs in the back. The new rear leaf springs was the latest change in September of this year. Therefore I can report which single change has which effect.
It depends what you want, I can say that this final setup is very good and enjoyable. Especially the rear leaf springs allow to set the rear dampers softer and the result is a good match. With the original much too soft leaf springs she bottomed out at each railway crossing and consequently I hardend the dampers. That was a bad compromise because a damper has not the function of a spring. Now, with the stiffer leaf springs she rides softer but much more controlled and without bottoming because I can afford to set the damper softer.
A side effect is, that the braking is much more effective and the car feels very secure. Another very enjoyable side effect is, that with the stiffer leaf springs you have an immediate acceleration feel when throttling. And the cardan will not touch the floor any longer under hard acceleration in first gear when your tank is full and you drive with a passanger.
Both positive effects are caused by the stiffer leaf spring. A too soft spring as the OEM Morgan one is has the ability to warp. It becomes an S. Thus the energy of the acceleration does not feed directly the wheels, but this energy is consumed first by deforming the springs. Thus the indirect time-delayed feeling when acceleration is wanted.
To summarize, my investments were not too high. Regardless if you want to invest in SSL front system (which I personally would not do so far), it is very important to have the rear optimized in any case. As I prefer the more classic and rustical route. And the fpur leaf 4 seater springs are way cheaper than a 5 link rear axle.