I haven't used a steering damper on a dirt bike but on road bikes they can be very intrusive at low speed. It feels like you're turning the wheel in deep mud. At higher speed they aren't noticeable except for the reduction in headshake.
If you search the forums for 'Scotts Steering Damper' you should turn up plenty of reviews. Personally I'd spend that money on a set of 17'' wheels with sticky rubber to make the most of your 100% tarmac days. These bikes make fantastic supermotos and swapping between the two setups takes minutes.