Im not sure how much travel you would need in both rear wheel bump (worst senario i guess, in roll i would have a guess that 45mm at the centre is plenty).
I would have thought this bump senario would be limited by correctly setting the rear shock abosrbers high speed bump or if you are really worried, I'd add a small/stiff/progressive bump stop in the centre.
This should allow enough freedom in roll and control the amount of bump movement (These bump stops could be incorporated into the shocks like Bilstien do).

If we are talking a 1200 coupe and your regulations allow, I would look at lengthening the front spring hangers as this will point the rear roll axis down further and produce better roll geometry (roll understeer if i remember correctly) and also increase the rear ride height.