I don't bother with slicks, no real need with Supersport tyres being so good these days, I gave slicks a try once and didn't feel any added benifit.
I know plenty of people that do use them and spend quite a bit of money on them, some of these people are quite fast while others aren't.

Some up to date figures to conjure with

MRO lap records for Brands GP

Powerbike Jonti Hobday 1m 31.38s (Club record is faster at 1m 30.79)
Supersport 600 Paul Seward 1m 33.02 (club recored id faster at 1m 33.01)

With Powerbikes running 16.5 inch slicks and Supersports 600s on treaded tyres I would say there is only just over 1.5 seconds in it on a track the length of Brands GP, the same applies to Cadwell full circuit on the Indy circuit its just over 0.5 seconds

So how much is that down to slicks or 16.5 inch slicks for that matter and how much to do with the extra power of the powerbikes