The crux is how much loss in spring damping/flex has occured due to the sagging. If it could be measured then matching the loss to a compensating extra could spring might work. Basically replace the lost effect. The difficulty is in working out the losses due to sagging so you can calculate the right spring wighting to replace it. Furthermore, how long would it solve the problem for?
The best solution is new springs or SSL.
Rog’s bump stops looks like a similar solution in some ways. As deflection increases the rubber is brought into play at extremes of deflection. Adjustment can potentially be made over time.
I had a few discussions with experts when my springs sagged. As some still know I took the opportunity and removed the lowering blocks, so a whole other topic for which I had made a thread last year.
Fact is, no matter how much a spring sags, the spring rate always remains the same as a new spring that doesn't sag. That is why it is very difficult to install an additional spring force. No matter what form, even the coil spring would add exactly 100% of its value, making it impossible to drive.
