Has anyone had any experience with surveying with a Suunto Tandem in the southern hemisphere?
I believe the Suunto _survey_ instruments are still sold in the traditional 5-zone system (the lower accuracy navigation compasses are now mostly two-zone, north and south). Certainly going from N-S hemisphere is the most extreme jump you can do. Declination is still subtracted / added as normal, it's the counterweight on the card to compensate from the off-planer drag of the pole that varies, and the alignment of the magnet (up + down) itself.
I guess you'd have problems both in the accuracy of a given reading (those last few degrees are where the force is minimal...), and likelihood of the card sticking.
As to how big these effects are, I have no idea.
Generally the correct compass regions are sold in the correct countries, so using a locally sourced set should put your mind at rest.