you need camber plate for the understeer
a slightly wider tires will not reduce it much. you can put a 225 on the 7.5" wheels but thats not going to magic.
a camber plate and larger rear anti-sway bar will do it.
the staggered setup from the Z4 has a same ratio so it will just understeer the same ratio, only a tiny bit higher speed due to the wider tires.
225 with a 245 rear will slightly reduce understeer but you can also play with tire pressure to reduce understeer. these are fine tuning of them, most people will not feel the difference.