Have used the SM81 for overheads etc but not taping; it's cousin, the KSM137, is my go to for cards on a budget (the 137's big brother the KSM141 provides switchable card/omni for not much more cash).
Given my results with the KSM's, I think you can make the SM81's work- KSM's roll off in the low end a bit more, but has a higher sensitivity rating (make sure you have a good preamp) and are a little cleaner overall. Nothing but positive feedback on my tapes; imagine you'll get good results with the 81's as well.