Yes, more weight in the boot will certainly help traction (same principle as engine, front or rear, over the driven wheels).
But as indicated, tyres with a good tread depth and the more 'block' in pattern like winter tyres are essential, for cars like the ZED. You can try dropping the tyre pressures (I'm just thing back to my trialing days when I would drop to around 10psi or lower), but with wide tyres on ice, as on heavily mudded surfaces, that can work against you as cutting through to solid ground where there is grip really demands narrow tyres, which we don't have.
So on balance I tend to leave the pressures on the ZED around 30psi at this time of year, but I am really tempted to try a pair of spacesaver wheels if I knew the majority of a planned drive was going to be on snow covered roads and you know you are unlikley to ever get up to speed. On the spacesavers you could also use chains if you really got into heavy snow but honestly, using the ZED with spacesavers and chains in such conditions has really got to be a last resort, but it would interesting to try it out