This is kind of cool, found a
spring rate spreadsheet by vorshlag. They measured the KW V2s for Z4 M.
KW V2 Front 10" length
1" 160
2" 324
3" 500
4" 694
spring rates: 160-164-176-194
KW V2 Rear 7" length
1" 227
2" 499
3" 917
Spring rates: 227-272-418 (after 3" starts to bind so rate shoots up)
So it would seem one could run as low as 224/336
Options are then: 224/336, 280/392, 336/448, 392/504. Part of me thinks I should go as low as possible to match the valving of the V3s. I was thinking of replacing the whole suspension with ASTs, but their 4(X)50s are taking forever.