ECSTuning just recently released a "complete" Sparta 6 piston Big Brake kit upgrade, front and back, for the E89 Z4 35i(s) models with color select.
This is the boys edition
https://www.ecstuning.com/b-sparta-e...123~dk/?pdk=Bw (The BMW OEM 6 piston option is better than this)
This is the MEN edition
https://www.ecstuning.com/b-sparta-e...123~dk/?pdk=AQ
This is the rear big brake upgrade assuming it is compatible with our ebrake
https://www.ecstuning.com/b-sparta-e...126~dk/?pdk=AQ