245/35: http://tyrestretch.com/10.0_245_35_R19/
255/30: http://tyrestretch.com/10.0_255_30_R19/
Obviously different tyres stretch differently etc but hopefully that helps.
Personally running that much stretch on a 10j would need plenty of low and a lot of spacing, and whilst I love a car that looks perfect when being parked up, you will lose a lot of what I think the z4 does so well (tearing up asphalt).
As I've opted to not run my other sub this summer, the rear will be getting a touch more low to sit flatter (and will then be going in for some corner weighting subsequently). Should handle quite nicely, I hope.