Whilst the electric steerign does provide some self-centering, it is mostly due to the suspension angles. Which you have changed if the new wheels are a different rolling diameter.
The front wheels are set up as a caster, like those on a supermarket trolley, so they try to follow the direction of travel. Being tied together helps fight their independence, so they don't have a mind of their own. The caster effect is created by the strut leaning back a little, and the centreline of the strut aiming at the road ahead of the axle line. If larger wheels raise the car, and the strut angle was quite shallow, it is possible that the strut centreline is now close enough to the axle line that it has little or no effect. Theoretically, if you move the strut line behind the wheel, it will try to turn around to face backwards.
I think we may have a clue as to why the Abarth uses different bottom arms. Abarth arms may sort this, but a lot of research would be needed to ensure it will, rather than just makes things worse. There may be other parts needing replacement as well.
A recalibration of the steering may help, but is not really the answer to the issue. If your wheels are significantly larger in diameter, your speedo will now read low, as the wheels will rotate slower, creating a risk of speeding without knowing. Hopefully you've informed your insurance company of the modifications, or they may refuse to pay if involved in a collision.
Good luck with this.
EDIT: The width of the wheel will change the geometry too, if its centreline is no longer on the centreline of the hub bearings. A wider wheel would need a different offset to keep its centreline on the bearings. But space may prevent this. If wheel centre is moved, this puts a 'bending' pressure on the bearings, instead of just a support pressure. May lead to shorter bearing life.
Bearings seem to last better these days, but owners of original Minis used to fit wheel spacers, and siffer significantly shorter bearing life as a result.
