The battery doesnt need coding in the computer for the e8x z4 chassis (that feature first came with the e9x I believe).
As long as you can start the car I cant imagine that the electric steerign column has problems with the power supply. The alternator can provide enough power.
Optima batteries are great but they're not a straight fit in the battery box I believe as they have a different footprint which isnt 1:1 compatible with the z4's battery clamping system, so that'll probably need some modification.
You can get inserts in the batterybox to switch between 80A/70A/60A battery sizes (I believe H5/H6/H7 size)
For a new battery (with different size) you also have to take into account that the positive and negative pole are on the correct side. The leads to the battery in the z4 are not very flexible and swapping them around sounds easy but it isnt. The positive lead has a stiff assemby connected to it that contains an explosive charge.
If its all over the road after having done work to the undercarriage, it probably needs an alignment (perhaps it has been aligned before with the worn control arms)