i use 30mm wheel spacers but i wouldnt use them with tyres that wide, plus the offset of the rim. i only use them with standard disco steels and freestyle alloys, both running 205's. i dont use them with my 235/85 diamonds as the rims offset is a little more. Any wider and you will munch wheel bearings i expect.
yes they do make a difference to stability, especially on my 90 which is rather quicker than standard (300bhp) and with the spacers and road tyres on the alloys its fantastic on the road! Although it hasnt been lifted at all and i run fox shocks.