Both my MC21s are without steering damper. Both slap a bit when they are being pushed but I don't mind it. My RGV used to tank slap a lot (as far as I can remember, or maybe I just rode that harder - joys of youth).

If you want to dial out tank slapping completely on your NSR, best advise is to probably get a different bike (ie not a sports 250)
_________________
Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30