I think it's fair to say that you've seen no increase in tyre wear Stealthy - as your (and mine when i was blown & spooled) rear tyres would doubtfully last more than a few hundred km anyway
By the nature of the spool (one wheel is breaking traction EVERY time you corner - moreso noticably on tight slow turns, obviously) the tyres HAVE to wear out at a [even if it is very slightly] increased rate to a single spinner, or a non-crazy-tight LSD.
If you're getting 20,000 km out of your tyres on a single spinner, then (for examples sake) you might see 18,000 km with a spool. - it's not like your tyres will go bald on your first U turn. - but who the hell gets 20,000km out of a set of tyres when there's a locked rear end?
- It's there for a reason!!
More to the point, Oken - if you're worried about tyre longetivity and don't want to be changing them too often, why are you considering a spool - the only legit reason to fit one is to be able to do some nice, controlled & predictable sliding
- and if you're worried about the tyre bill, this is probably not the best option!