John - I went to Mulfab after a fellow TM'er had gone this route. Mulfab have sold a lot of rear springs to correct the weak versions. I went for the 4 seater springs because I want decent luggage capacity without bottoming.
(Other spring suppliers are available).

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4