GB,
The shocks off a 2003 MCS should be the firmer version. If I remember correctly the production change was in 2004 before the facelift. There was a thread about this mentioning the change in response a to Consumer Reports survey. Please search for that thread because I'm going off memory!
The SS+ springs will be about 30 lb/in stiffer than what you've got now. The free lengths of the different OEM springs varies. This was to accommodate the weight differences between the different options & overall wheel/tire diameters. For 03/2002 on Gen-1 R50 & 53 models there are three different SS+ front part numbers & five different rear SS+ part numbers. Rates should be the same for SS+ but the free lengths vary. There is a definite possibility the ride height could go up. I did this same swap on my R50 with 15" rims & the wheel gap was pretty unsightly
If the price is right, it would probably be worth it for the shocks alone. If you do your own work, It might be worth trying the Springs. With your 17's they might be OK.
It could help Mini owners wanting to swap OE springs if a a sticky thread was started where owners could list the build date of their cars, factory options & color of the paint stripes on their OEM springs. We could then cross reference that data to the OE part numbers. There's also a lot of confused info about JCW springs out there. It would help if we included them in this data collection.
It's very easy to check the paint stripe when you take a wheel off & the build date is right there every time you open the door.
