In my experience messing with offset, especially on a well setup car, is seldom beneficial to handling.
This is in part confirmed by a number of people on here having bought spacers then put them up for sale not long after fitting them.
I don't deny that a wider track looks good, but making such changes usually comes at a price.
That said, altering the rear axle should have a much smaller impact than altering the front where there are steering angles, etc. I know my Puma was very sensitive to changes in offset at the front, but changes at the back made no apparent dynamic difference.